I wont run less than a 50 on everyday cars ever again so many buckled rims not worth the hassle

They didn't have 245/50/17,s here in Australia otherwise I would have tried them

They also didn't have 235/50/17,'s with the same tread pattern as 255/45/17's otherwise I would have just gone smaller on the front after all that's what causes the tram-lining so it was the 235/50/17's after a huge amount of research very happy with that risky choice as Doug is also

It's very hard to stray from what the factory says I.E. experimenting but in the end so many have gone the 235/50/17's now its a forgone conclusion

I would love to see what 245/50/17's do to the speedo though as the 235/50/17's did move it up closer to the gps accuracy on

My wife's Jeep GC went from the specified 275/45/20's to smaller 255/50/20's the speedo is spot on and the ride is also so much better also don't scrape the rims when backing in next to the curb unlike my new XJ8X350 I have trashed the rims beautifully now with the 35 and 40 profile tyres

 

Can't speak to accuracy, knowing not where your speedo stands today, but the effect of a tire change on actual ground speed is not too hard to work out:


Ground Speed in MPH= The QUOTIENT of the PRODUCT of Tire Outside Diameter, Engine RPM and Final Drive Ratio upon constant 336 (336.13523981xxx for exactonoids)


well...that hardly makes sense to me...and I wrote it!!!


MPH=[OD X RPM X FDR]/336;


Where OD is tire mounted diameter in inches,(look at TireRack.com for your particular tire)
RPM is engine from the tach and
FDR is the final drive ratio accounting for transmission and rear differential. (Remember, most xmsns these days are not 1:1 in Drive, but some Overdrive ratio of .7 or so.)
336 accounts for all the constants involved in the derivation including pi and the conversions from inches/min to MPH.


Your speedo, of course, will report the same speed at a particular RPM regardless the dia of your tires.

 

Actually, I find it very easy to do ! Jaguar originally used the old Pirelli P-Zero tires on the XJR/6. Hugely expensive, notoriously short treadlife, and bad tramlining. Getting away from those is a very easy decision for me. We have more choices, and better choices, than Jaguar had 20 years ago.

But, yeah, changing tire spec/type it isn't something that I suggest be done *****-nilly, without knowing the pros and cons of your decision.


Cheers
DD

 

Yeah, I'm heavily leaning toward the 50 series already as well at this point.


Interesting your comment above about the 235s moving the speedo "up" closer to the GPS suggests a little more evidence that the XJR speedo is indeed probably still calibrated for the regular XJ6 wheel/tire spec of 225/60-16, which I was wondering about a few posts above.

If that *is* true, then the following in ascending order should be the closest size to match the XJ6 225/60-16 whee/tire:

225/60-R16 vs 255/45-R17 Tire Comparison - Tire Size Calculator

225/60-R16 vs 235/50-R17 Tire Comparison - Tire Size Calculator

225/60-R16 vs 245/50-R17 Tire Comparison - Tire Size Calculator


Looks like the 245/50-17 as spec'd for the 17 inch wheel XK8 is probably the right fit.

Makes one wonder if the slightly "undersized" 255/45-17 was spec'd for the XJR to give it a slight acceleration boost from marketing perspective to compete with its competitors of the day?

.

 

The overdue rain finally came a few days ago. I'm happy to report 'no ill characteristics' in the wet weather performance of these tires, and I pushed it a bit....no crazy man stuff.... thru some twisty bits.

Cheers
DD

 

Well, I had been looking at the Pirelli P7 Cinturato, but Discount Tire make me too good a deal on a set of Michelin Primacy MXM4 (not the Pilot MXM4 or HX MXM4) in the 245/50R17 size.

This tire has been around for a while, and I think Michelin is phasing it out, but it has great reviews:

http://www.tirerack.com/tires/tires....l=Primacy+MXM4

As I mentioned, I chose the 245/50R17 size since it was the OEM size that Jaguar used on the 97 XK8 on the same sized 17x8 rim. And in this size there are a lot of longer lasting options than in the 255/45R17 size that is standard on the XJR.

My initial impressions are that the tire rides great and is very quiet, but to be fair the Kumho ASX that were on the car were so badly worn and cupped that any new tire is going to impress.

But the Kumhos only lasted about 15-20k before they were toast, and I dragged them out to about 25k miles before this change.

So we'll have to see how the Primacy MXM4s last but I think they'll be a good choice based on the reviews.

So far, the tramlining does seem reduced too. I ran the car through a section of highway that was very sensitive to tramlining, and it seemed gone or at least reduced dramatically.

Time will tell as I drive it on different types of roads.


.

 

Curiously, should also add that the front end 'groaning' at low speed (under 15 MPH) has gone away.... that I thought was surely my front wheel bearings. Hard to believe worn tires would cause that noise, but it must have been.

.

 

I have been following this thread with great interest, and by chance came across my original sales brochure from 1995. On browsing through it, I came across the following quotes, which seem highly relevant: "8x17 inch, the largest-ever on a production Jaguar" "The aggressively low-profile tyres, the Pirelli P Zero being specifically developed for the XJR, give total conviction to the ranges sporting stance" Clearly much has moved on in the intervening years, and no manufacturer would be bragging about 17 inch wheels and 45 profile these days, but it does highlight that the spec was seen as being at the leading edge at the time. The claim that the P Zeros were developed specifically for the XJR can only refer to the tyre in the particular 255/45 17 size, as the tyre featured on many other vehicles of the day, notably products from Maranello, but the comment does demonstrate Jaguars desire to enhance the car's street credibility.
I was fortunate to have both the x300 and the x308 XJRs from new, and both were delivered on P Zeros. It is true to say the x300 tramlined a little more than the x308, but in truth, it was not the defining characteristic of either, and certainly did not make either difficult or dangerous to drive.
Fast forward many years, when I acquired my current x300, it needed new tyres. I was intent on fitting P Zeros as much for originality purposes as anything else, but at the time they were not available, so I opted for Dunlop sportmaxx Gt in the correct size and speed rating. The car drove well in these, notably in the wet, but my joy was relatively short lived, as the rears needed replacing after 8000 miles of sensible driving. Having replaced like for like, the fronts lasted till 16000, which I would still not consider a high mileage. When looking to replace them, I was introduced to "MO" spec tyres. This is a Mercedes Benz rating, and all Mercedes are supplied with MO versions of the relevant tyres. It is not entirely clear what the significance of the MO rating is other than the tyres are supposed to be "better" And yes, they are more expensive, but only about £10 per corner. I fitted the same Dunlop sportmaxx Gt tyres, MO spec, and am currently through 16000 miles with the same good performance from the tyres, and the same driving style. It tramlines a bit, but no worse than when new.
In thinking about tramlining, it is worth remembering that these cars are sensitive to wheel alignment, particularly at the rear, and if this is out tramlining could be but one consequence.
In otherwords, don't just blame the tyres.
It is not uncommon nowadays to see manufacturers specify specific makes of tyres for their vehicles, particularly the higher performance variants in the model range, and car magazine road testers often discuss the different handling characteristics which result from changes of brand of rubber. The lesson here is that not all tyres suit every car, and the XJR is no different in this regard. Top quality properly sized tyres fitted to a 100% perfect XJR could still produce a sub optimal outcome, and to a degree, vice versa. This long after the car went out of production, there is bound to be a degree of art rather than science in determining which modern tyres suit best, which is why this thread is so helpful. That said, I would be very wary of fitting non standard sized or differently speed rated tyres. Much has been written in the thread about how to maintain the circumference of the tyre, and so speedometer accuracy, but by far the bigger issues are size of contact patch and tyre carcass quality (for which speed rating is a good proxy). Ask your insurer's opinion about fitting 245 instead of 255 profile, and fitting a tyre rated below the manufacturers recommendation.

 

Good posting



Indeed. Hampered by the reality than tires are too expensive for most of us to do much experimenting.




In the USA, a least, we have 'load rating' which (we assume) is another indicator of the tire's overall strength....perhaps more useful to than speed rating.




Not an issue in the USA but apparently worth considering elsewhere, as it is mentioned often.

Varying from spec does take a bit of thought but it appears the safety risk is quite small unless a person outright abandons common sense. I suspect the reason that USA insurance companies don't care about tire specification is that only .5% of all motor vehicle accidents in the USA are tire-related. And, of those, the majority are relates to tire *condition*....under inflation most particularly.

The tire spec issues has been of much concern for owners of the older Jags with 15' wheels. For a number of years now it has been virtually impossible to find 15" tires in the correct size and speed rating. And, frankly many USA owners were using non-spec tires even when correct spec tires were still available. Hopefully they know to avoid high speeds. But, in any case. I'm not aware of any sudden increase in crack-ups of older Jags as a result and there really wasn't any reason to expect any....as darn few are ever driven at 125-140-150 mph. Not in the USA, at least.

Anyhow, I'm not disagreeing with you . Varying from spec does sometimes involve taking on some risk and should be given due thought beforehand, and actual use/driving conditions should be part of the thought process. If a person is unwilling to do so, or simply doesn't understand....then sticking with factory spec is the safest choice.

Cheers
DD

 

The Michelin meets the load spec, and is rated for 138Mph if I recall, which is less than the original Pirelli but close enough for my purposes

Most reputable US tire shops will not install a tire that does not meet the manufacturers load rating.

BTW, the 245/50R17 looks like a perfect fit IMHO ...and as mentioned, is the size specified for 1997 XK8


.

 

Tyres in the UK have a load rating also, and it is important IMHO to at least adhere to the manufacturers spec on this too, but it is more related to the weight the tyre can support, and you can obtain tyres with a high load rating in all of the low speed ratings. For example, Van tyres typically have high load ratings, but lower speed ratings.
To obtain a high speed rating, a tyre has to be able to cope with the extreme centrifugal and other stresses at high speed, and therefore has to be particularly well manufactured. The P Zeros originally fitted to the XJR were Z rated, suitable for speeds in excess of 150mph, and even if you are not planning on driving at this sort of speed, the Z rating means that the tyre has a superior build quality which should yield benefits across the speed and handling range.
If I ever were inclined to compromise on tyres, it would probably be on a lighter and slower car than an XJR (for example, an XK8), but even then, I would be loathe to fit a tyre with a narrower contact patch than specified by the manufacturer: we can debate speed and load ratings, but less rubber on the road is less safe in most everyday road conditions.

 

Yes, but on the X300 chassis, for example, Jaguar used at least three different sizes that I know of: 225/60x16, 255/45x17....and....I'm having a brain fart...... 225/55x16? And at least three different tire designs. And I'm betting that none of the designs selected by Jaguar 15-20 years ago still exist in the same form today.

Heck, my XJR was built with 255/45x17 but I drove it for probably 100,000 miles with 225/60x16 tires. Nothing horrible happened, I assure you . In fact, since the tramlining was 100% eliminated there's a very good argument that the down-sizing made the car substantially more safe. I wouldn't let my teenage daughter drive the car with the 'Jaguar specified' tires but had no qualms whatsoever about letting her drive the car when I down-sized/changed spec.....'reduced the contact patch'.

I know what MY requirements are. I refuse to be a slave to specification when I know darn good and well I can do a better job of selecting tires than Jaguar can. I have no reason to believe that the Jaguar tire-deciders have my best interests in mind!

As I previously mentioned, if uncertain then sticking with what the factory says is a safe bet.

Cheers
DD

 

Remember too that between manufacturers of the same "size" tire, the contact patch and "tread width" can and does vary significantly.

I did a lot of research (almost too much, analysis paralysis) on this when looking at the 255 versus 245 section tires and it varied widely at times. It also changes based on the rim width applied.

Tirerack has some formulas on this that show a 245 tire on a 8" rim versus a 255 tire on an 8" rim actually aren't that far off as the size of the rim pinches the 255 tire a bit. They use the term "measured rim" as the rim width the manufacturers use to state their specs such as "tread width" and all used a wider rim than 8" for the 255 as the standard.

I honestly don't think taking away 10mm in section width (not necessarily 10mm in tread) is going to make a significant or even measurable (by normal people, driving on the street) difference.

I think the 255 was mostly "for show" to tell you the truth, bragging rights

But if one is uncomfortable deviating from the OEM spec, please don't let me convince you otherwise. It is your car, your peace of mind and your safety

I'm just offering my observations and opinion.

.

 

Doug, Al,
I am not trying to rock the boat on this subject; quite the reverse, and it seems there is much we agree on here.
There is however no getting away from the facts:
-Yes, Jaguar fitted different tyre sizes to the X300s. These however fairly directly tracked the performance levels of the respective models. The XJR was, by some margin, the highest performing vehicle in the range, and had tyres to match.
-Yes, Jaguar wanted bragging rights, and the text I quoted from the sales brochure clearly shows this. That doesn't alter the fact that the XJR was specifically designed for the particular rim and tyre sizes, in the same way as it has specific damper and bushing specifications, etc. Try comparing the tyre sizes to other equivalent cars of the day, and you will quickly see that whilst bragging, Jaguar weren't "gilding the lily"
- Taking away 10mm of tread width would approximate to a 5% reduction in contact patch size, or, put differently, 5% less grip with the road. I don't doubt that the car can be driven indefinitely without incident on the smaller tyres, right up to the point when the unexpected happens, and the extra grip would be kind of useful all of a sudden. In similar vein, would you use brake pads that you knew to be 5% less effective than specified.
-Yes, different rim widths and offsets (and tyre pressures) affect contact patch size, but we are I take it talking about a like for like comparison on standard wheels?
-yes, reducing the width of the tyre would reduce tramlining, as it reduces the contact patch, and hence grip. Running the tyres at higher than specified pressures would achieve much the same, and be inadvisable for the same reason.
- If a car tramlines so badly that a teenager could not drive it safely, then I would respectfully suggest that something else must be amiss with the car or the tyre/car combination. I would emphasise Doug that I say this most respectfully, as I have read many of your posts, and know how particular and painstaking you are in your approach to maintaining your vehicles.
I can assure you from personal experience that these cars did not tramline badly when they were new, and certainly not to the point where that were in any way hazardous, in addition to which, my current X300 on Dunlop sportmaxx Gt tyres of standard size and specification does not tramline to any meaningful degree, especially when compared to other high performance cars.
Please be assured, I am not at all shy about departing from the manufacturers specifications where there is a good case for doing so, for example with brake fluid, where there have been significant advances over the years, but I cannot find a way to persuade myself that reducing contact with the road is a good idea in any car, let alone one as heavy and powerful as this.
I am enjoying our discussion, and hope that I am not tempting fate and end up tramlining through a hedge tomorrow!!

 

Gotcha, no problem at all, always good to discuss different perspectives. I certainly don't know everything, and don't profess to be a "tire engineer" or even automotive engineer



All reasonable observations, but I'd also offer that the Pirelli PZero "designed for the XJR" back in the mid 1990s as the best performing tire for a vehicle like the XJR did not have to compare with tire technology of 2014.

322HP was a lot in 1994, but tire technology has come a long way.

Would Jaguar still spec a 255 tire if the X300 XJR were released with 2014 tires? I can't say as I'm not an automotive engineer, but something to consider.

And I am going to go out on a limb and say that there isn't a direct/linear relationship with 5% of contact patch translating to 5% of traction loss. There are just too many other variables to consider such as suspension, tire composition, road condition (wet, snow, dry, temp), air pressure, new technology, etc...

Also, along with technology advances in tire performance, "more" is not always better. A wide tire is subject to physics just like everything else, and as the tire is widened, the PSI on that contact patch is distributed. Sometimes a narrower tire with different grip performance may perform better in some respects.

But I of course for example can't objectively evaluate how the 245 Michelin Primacy MXM4 performance differs (better or worse) than the OEM 255 Pirelli PZero as I don't have the time, money or equipment to do so. But I can say that in my case the Michelin meets the OEM spec (except for speed rating by one category, which is NOT a concern ) and has excellent performance reviews, especially being excellent in the wet which a summer HP tire like the PZero does not. And the MXM4 is good in light snow too, where the PZero has no rating for snow.

Oh and the Michelins (supposedly per their warrantee) won't wear out in one year for me. And THAT was a huge consideration when looking at my options in the 255 range. All of the tires I looked at would likely have to be replaced in one year of 20+k mile driving for me. And as you know, they aren't cheap in the 255/45R17 size

The new ones also drive nice

Are the Michelin Primacy MXM4s the best choice? I don't know. But they seem to be working out well so far, and they fit my need.

As far as tramlining, I also think some of that issue is subjective. Different drivers are going to be more sensitive to it than others. I noticed mine much less after getting acclimated to the car over 5 months. At first I really thought it was 'extreme' never having to deal with it before. Five months later, I considered it negligible, although still there in extreme cases of road rutting.

.

 

I'm pretty sure that the XJR wasn't designed around the wheels and tires. I think Jaguar had an idea of how they wanted the car to steer and corner and selected wheels and tires that they thought would best flll the bill







Oh, sure, extra grip is always nice . But I dropped down to the size used by the standard XJ6 and XJ12. I guess you might say, then that the my XJR then became as unsafe as they were using the same size tires. !




Well, I've owned mine since just 30,000 miles and it was tramlining badly even then.....and a seriously doubt that much wear had taken place. And, yes, I do keep things right up-to-snuff. Nothing amiss at all.

Given the many complaints about XJR tramlining over the years I doubt that my experiences are particularly unique. A car that suddenly darts into the next lane of traffic at 70 mph...with no input from the driver....is no place for an inexperienced driver who might over compensate.

However, OTOH, tires themselves are the biggest contributing factor it seems....and I can vouch that some are much worse than others on the XJR. My experience is with Goodyear Eagle-somethings (memory fade) and BF Goodrich KDWS ...and the Goodyears worse tramlining than the BFGs.

Never tried a brand new set of the P-zeros. Far too much money for far too little tread life. And, besides, you can go back and read contemporary reports where road testers complained about the tramlining with P-zeros. So I had little inclination to try them.






If I thought Jaguar did a really good job of selecting tires I wouldn't be so inclined to change them. As it stands, I don't think they did....so I make my own choices. it might be that the 255/45x17 P-zero was the cat's meow when the XJR/6 was in the design and testing phase...but that was a long, long time ago !

Remember the famous Dunlop SPs Jaguar specified in the 70s? And the P5 Pirellis jaguar used in the 80s? I'll wager a week's pay that, in the day, owners couldn't wait to change to a different tire!

Much more recently remember the (2004-05?) base model XJ8s (USA market) with the notoriously low tread life standard tires? So many complaints that Jaguar changed spec to a lower speed rating and higher tread life rating....and reduced the top speed governor. They simply selected the wrong tire for the intended market/buyer.

Enjoying the discussion also. All's well

Cheers
DD

 

Al,
You make a good point about advances in tyre technology, and whilst I don't have any figures to hand, it would seem safe to assume that the grip lost by reducing the section of the tyre would have been offset by now by the improved grip properties of modern rubber, leaving you with at least as much grip as Jaguar intended. Not as much grip as a 255 section tyre on modern rubber though.....
I agree with your comments about wider not always being better. Snow and standing water come to mind as situations where it wouldn't be, but I guess neither an XJR on 245 nor 255 would be a good choice in those situations!
I should probably clarify, that I am not a poster boy for P Zeros. They are now available again in the correct size, but I am not planning on fitting them. They are meaningfully more expensive than even the MO spec Dunlop Sportmaxx GTs I am using, and there seems to be a general consensus that they would not see the far side of 10K miles. Whilst they score on originality, and they do look good on the car, these are not the criteria to use for a daily driver, so not for me.
Michelin make good tyres, and I look forward to hearing how yours play out in real life.

 

Bear in mind that maximum tire performance is only relevant for those driving at 10/10ths.
And completely irrelevant to those driving at 11/10ths (they don't live long ).

Also bear in mind that the XJ and XJR could probably reach the same speed neighbourhoods ...
just at different rates, and then need to corner at terminal velocity.

The conclusion would then be, if one were to trust Jaguar engineers, that the 225 spec
on the XJ was perfectly adequate at top speed.

Now, in reality most drivers never live in that neighbourhood. So, they have a cushion.
And if they drive in rain and snow, they are probably better off on 225's.

Finally, the XJR received the 245's in part because of "image", just like the race to
truck size rims in the current models. Just because 20 inch rims are in vogue does
not make them particularly necessary or even useful.

 

Doug,
Thanks for your reply.
Having re-read my post, I can see that my choice of words may have led you to think that I was saying that the XJR was designed "around" its wheels and tyres, although that is not in fact what I said or meant. I am sorry for any confusion.
Whilst as a starting point I would share your cynicism regarding vehicle manufacturers fitting whatever parts they have lying around in inventory, I do not believe that the XJR was a case in point.
It was an important model in an important new model range, the first developed fully under the auspices of Ford. Jaguar had never fitted a 17" wheel to a production car before, and so did not have any lying around gathering dust. The wheel was sourced specifically, and as it turned out uniquely, for the XJR. It was not never fitted on any other model, nor was it available as an option (nor were the other upgraded suspension items from the XJR).
Given it was a new wheel size, new tyres were needed, since none were in inventory. Whilst I think the claim in the brochure that the tyre was developed "exclusively" for the XJR is rather grandiose, it is I think factually correct with regard to the specific size in question. It is at this juncture worth remembering that the P Zero Assymetrico (to give it its Sunday title) was hot property at the time, and featured on all sorts of exotica, so it was a highly credible choice, and would have done no harm to sales prospects.
Perhaps a better way therefore for me to have expressed myself would have been to say that the wheels and tyres were an integral part of the design, and I would certainly stand by this statement. I would happily wager that more time and care was taken with these aspects of design on the XJR than any other model in the X300 range.
As an aside, but actually quite relevant to our discussion, I have a friend who bought a new Ferrari 550 back in the day. Waited ages for it, and drooled over the brochure to console himself. The car in the brochure sat on, you've guessed it, P Zeros, and really looked the part. Fast forward to its arrival, and he is crestfallen to discover it is sitting of Michelin Pilots.
Kicks himself for not actually specifying the make of tyre, but can't wait to drive the car, so off he goes. Some time later, (but not so many miles!) track day use has put paid to the Michelin Pilots, so he puts the car into the Ferrari dealer to have a new set of tyres fitted. "P Zeros please" . "Sorry sir, this car has the Fiorano handling pack fitted. We are advised by ferrari not to fit any tyres other than Michelin, as the handling will be upset"
This lends support to the view variously expressed in the thread regarding the importance of the right combination of tyre and vehicle condition being critical, which I suppose is obvious anyway. As the wheel sizes increase, and the rubber gets wider, but lower profile, the margin for error or forgiveness must get smaller, not just on Jaguars, but all cars.
I am not sure that I follow your logic in your comment about dropping down to the XJ12 tyre size. I am sure they are perfectly safe on the XJ12, and have not suggested otherwise, but the XJR is a different kettle of fish entirely, and is designed to perform in a materially different way from the XJ12, as road test reports would unanimously bear out. Wheels and tyres are a key part of that equation.
Finally, you made me misty eyed when you mentioned the Dunlop SP sport! I thought that tyre looked so good, when as a schoolboy I used to ogle XJ6s in the showroom. The tyres seemed just so much wider and different from anything else. They looked great on other cars too, most notably the Triumph Dolomite Sprint, which I had back in the day. They were great in the wet, and quiet, and if only they had been made of liquorice, they would have lasted longer! And yes, I did change them at the first opportunity, but having bought a Sprint a couple of years ago, I will not rest till I have a set on it. They are being remanufactured now, albeit not yet in the correct sprint size. They car just doesn't look right without them... Am I spending too much of my life thinking about tyres???!!!

 

The XJR suspension is fundamentally the same as all the other X300s. No changes in layout or design. It did get some firmed-up springs and shocks (as did the other models if the sports-pack was ordered), a larger front ARB and the addition of the rear ARB (again, available on the others).

Of course is natural for a manufacturer (or even an individual) to go to a more performance-oriented tire when making such changes.

We know what tires Jaguar selected for the XJR. For the other models with the sports suspension (which, again, was essentially the same as XJR) a 225/55x16 tire was used....which is scarcely any change at all from the standard 225/60x16 in terms of size although I'm such a sportier design was chosen. So, from my perspective, I'm inclined to think that Jaguar didn't feel the 255/45x17 P-zero wasn't essential (or even ideal) from an actual driving standpoint.....which I can attest to from personal experience






Sounds reasonable to me: the size was unique to Jaguar but the P-Zero design itself was not.





I'm sure Jaguar wanted a high performance tire/wheel combination to go along with the image they were trying to create and, yes, I'm sure they put some thought into the decision. Where we disagee, obvously, is "how much" thought! I obviosly think it was much less than you do.




Which proves nothing, really, except the that the fellow at the dealer is a slave to factory spec....as are most dealer employees. It's easy. No thinking or discussion required. For all we know the customer know may have preferred the way his 550 drove on something other than what Ferrari deemed best! Surely even Ferrari will admit that whatever tires they decide on are a compromise just as any such decision would be. Well researched, perahps, but still a compromise in some way or other. There's no certainty that every customer will find the tires suitable in all cases or circumstances.

For example, a person in my neck o' the woods might prefer something that offers better wet traction than what the factory installed. The next fellow might want something that offers longer tread life than the factgory installed tires. Or the 550 fellow, since he wanted to track the car, might've preferred something with a tread design and compound aimed specifically for track use. I'm sure a tire expert (rather than a dealer employee who just parrots the factory recommendations) could have assisted him well.



Critical? Are you sure?

As mentioned, for years I drove my XJR on 225/60x16 tires rather than the 255/45x17 tires. I was perfectly happy and the car behaved perfectly well.

I do admit that a small amount of steering crispness was lost. And, most likely less traction if pushed to the absolute limit. (I'm fairly aggressive and enjoy the twisty roads and never reached that point, to be honest. I'm tempted to say you'd have to be a madman with hair on fire!)

I can't really describe the 255/45x17 as being 'critical' to the safety or enjoyment of the car. That size, in the right design (not P-zero!) might well optimize the steering/handling for at-the-limit drivers, I fully agree. And, that size in certainly in keeping with the image that Jaguar was trying to convey as well.

Of course, 'optimal' is nothing at all like 'critical' and image is, well...something for each of us to decide on our own .



Well, you were mentoning tire size and the smaller contact patch as a safety consideration. I was saying that 225s are no less and no more unsafe on an XJR than they are on the XJ6/XJ12, that's all



Agreed! Unless you ordered the Xj12 with sports suspension....then the difference becomes narrower


Road and Track tests reveal that the comfort suspension XJ12 and the XJR give near identical results on the skip pad.....that is, maximum adhesion was almost identical. 'Twas thru the slalom that the XJR really showed it colors, not surprisingly.

Anyhow, yes, tire are part of the equation, no argument.

In the end we're at the same place. You clearly put a lot more credence in factory recommendations than I do . My jaded view my comes, at least partially, from working 30 years at car dealerships. Car manufacturers always, always, ALWAYS wag their finger at us and say "We know what best for everybody. Never disobey. Trust us. You'll be happier".

Well, in fact, sometimes they don't know what's best for everybody.

A more complete comment from the manufacturer would be: "With various considerations in mind we designed and engineered your car with built-in compromises that we felt, at the time, were the result of good decisions on our part. For a variety of reasons, which often serve our needs more than yours, it is much easier and better for us to insist that you just be good lads and follow our directions with no varying. You are not smart enough to understand these things. Thank you. Jaguar Cars, Coventry England".





Cheers
DD