This is incorrect and the factory did not offer this as an option. Rears are supposed to be 285/30x20. Also check the rim width on the rear wheels. They should be 9.5 wide which is a full inch wider than the fronts, the fronts being the standard 8.5 inch width on all other XF rims. You may have to take a wheel off the see the numbers as they are often stamped or embossed inside the rim, on the back of a spoke or on the edge of the inside of the hub or on the hub face between the lug bolt faces.

Usually the rim size 20 x 9.5, the profile "J" is likely, and an ET number (offset) will be somewhere on that rim.

A quick and dirty way to check is to look at the fit of the tire. 255mm is 25.5 cm which is 10 inches, that is the tread width, plus or minus. The rear tires in this size would be a tight fit on a 9.5 inch wide rim and the sidewall will look tucked in compared to the front tires in the same size but fitted to 8.5 inch wide rims. If the front and rear tires look as if they fit the same then you have been deprived of the correct sized rims as well as tires, on the rear.

 

My 10 SC is as you say 255 35 20 on the front and 285 30 20 on the front. Assume there is a 1" wider rim on the rear.

I am still wondering if I could have gotten 18" rims to fit over the front brakes but the calipers look to be within 1/2" from even the 20's.

 

For those who bought the car for hp it makes all the difference because they'll us the power and notice the difference. For practical reasons you could buy a Honda Accord and not care at all.

So you think wagon wheels would be better suited for the car then? That could save us all a lot of cash. Wider tires were fitted to the SC and R to get the extra power to the ground. Larger brakes were fitted to dissipate the heat generated when you use that extra power and need to slow down. If the brakes were too small they would warp and turn into paperweights. Race cars are lighter and won't generate near the amount of heat a large 4 door car will not to mention race brakes are probably carbon and can dissipate heat better anyway.

I did some research and there is some people that take your stance but here is some good real world data (too much in my opinion... lol) that shows the contact patch is indeed larger for wider tires:

Fact or Fiction? Tire contact patch and air pressure.

 

I belive his point is for 99.9% of driving that everyone will do bigger brakes and wheels are simple a visual thing. Unless you're tracking the car hard then you'll never see any benefit of the larger setup over what the base car offers.

 

The link posted above may be interesting but it isn't relevant to the issue of contact patch area being independent of tire profile.

In reality, wider tires are worse for traction than narrower tires. Wider tires do not "get the power down" better. And don't use the drag car tire example or we'll be here all week.

Elementary physics can be used to prove that tire profile, all other things being equal, has no effect on contact patch size.

A wider tire on the same vehicle will permit higher side forces (cornering grip) to be developed at lower slip angles. That's why wider tires are fitted, assuming it is not just cosmetic. It has nothing to do with accelerating or braking, within practical limits (the web page talks about tire temperature only to go on and ignore it, odd since tire temperature is critical to grip, ironically, wider tires tend to run cooler so generate less acceleration grip than narrower tires most of the time!).

A tire with stiffer sidewall will have a different (smaller) contact patch area to the same profile tire with a softer sidewall (one reason runflat tires are so hopeless). All other things being equal is an important qualifier.

The poster's understanding of braking physics s also intuitively attractive and wrong. Racing cars may be light but they are also fast. Brakes absorb and convert momentum into heat. Momentum is calculated as mv where m is mass (weight to the layman on planet earth) and v is velocity. A thousand pound racecar travelling at 200 mph will need brakes as big as a 4,000 lb car travelling at 50 mph, all other things being equal. Formula 1 cars weigh less than 1,000 lbs and have tiny brakes tucked into the tiny wheels they are permitted under the rules, so they will look like proper race cars (styling, actually, as Le Mans racers have had bigger wheels, lower profile tires and bigger brakes for years).

Large brakes on a road car are pure marketing hype. Standard brakes on an XF are more than up to the job of stopping the car. Put the same tires on my lowly Luxury XF and my car will stop shorter than an XFR, it's lighter.

If this were not true then children would be able to walk on slippery surfaces an adult could not. Friction is described by a coefficient (unit less) for a reason.

 

I have a 09 XF supercharged and it came with the20" Selena wheels that look exactly like these Nevis wheels on this XFR :Used 2010 Jaguar XF XFR For Sale | Huntington NY .

But mine came with the 255's all around too. I find this weird????

 

I started this thread and I have the 255's all around. The rims front and back are exactly the same. My ride doesn't suffer and I can rotate the tires since they are the same size. In addition the replacement cost will be lower.

Win, win all around.

 

Volans are staggered fitment, 255 front 285 rear, unless your Volans are replica's ??

 

How does a 255 tire on a 285 Volan fit? I see no difference on all fours?

 

I would guess that someone bought a pair of 255 Volans and put them on the rear in place of the original 285's so that they could use and rotate single size tires.

 

So what happens if you go one size bigger all round. Does it give you a little more curb protection? What are the negatives?

(9.5 x 20) 285 x 30 x 20 to 295 x 30 x 20 = diameter change of 6mm or .88%
(8.5 x 20) 255 x 35 x 20 to 265 x 35 x 20 = diameter cahnge of 8mm or 1.17%

 

Clearance for steering lock is likely an issue with wider front tires. Some UK owners report rubbing on fender liners with some brands of proper sized tires.

I noticed a tendency in NZ to fit larger than stock front tires. Not very common but common enough for me to notice. Mind you, reducing understeer would be useful in NZ!

 

Thanks for that. Probably won't happen anyway as 295 x 30 x 20 are limited and pricey here in NZ and I'm not going to pay $1200NZD a tire when I can get Falken 452's for $400!!

 

I'm beginning to think about ordering my summers as when I put the snows on the oem dunlops were thoroughly done. The Michelin Pilot Super Sports sound like a great way to go. Anyone think prices will go up between now and May? Discount tire has free shipping to Canada so will probably go that way but trying to decide if I should order now or wait a few months. One advantage currently is they are offering a $70 visa card with an order by eod today.

 

Buy now. Tire pricing will follow demand which will increase dramatically when people who put away thir summers without really looking at them dig them out and see they are shot. This actually happened to me with a set of winters. Fortunately, I had a surplus set I was saving for just this eventuality. Took me a week to get an appointment at the tire shop to fit them though. Plus, think opportunity cost of money. If you have the money and need the tires anyway you are " losing" only the interest you might have earned ( at under 1%). The manufacturer and distributor will earn the savings of interest costs to them at least at prime. Thats where the room to discount comes from. Buy when the production runs have just been completed. I got CDN$40 cash card on my purchase of continental winters last fall when supply peaked.

Good choice BTW. Everybody who tries the SS is totally impressed. However, Evo magazine in the UK just published their ultimate tyre test and put the newest Continental Contisportcontact3 at the top of their heap. The Michelin PS3 came fourth. However, the Michelin SuperSport is reputed to be the best available in that category. Ie better than the PS3. Evo doesn't say why they didn't test the SuperSport.

 

You cost me this morning Jagular. You pushed me over the edge. Order in, Super Sports. $1660 to my door less the $70 Visa promo. Pretty good for Canada.

 

At least you'll be enjoying them before Victoria Day. Out West here in Calgary my winters stay on until after that weekend. Mind you, last year I didn't have to put the winters on until December. But I still have the stock all seasons. With your SS summer tires you will have to change to winters when ambient temps drop below 7C to get decent grip, snow or no snow.

Let us know how you like those top level Michelins. They are supposed to be the ultimate street tire currently available.

 

Probably throw them on mid april. Looking forward to some grip. I'll definitely post a review.

 

I believe that Michelin PS3 are the UK/Euro name for the Pilot SS. I love mine, no complaints at all. Always had good experience with Contis too, but they never appeared as an option when searching for XFR sizes. The Evo test also test tyres by Vredestein that are very well thought of in UK performance car circles, but aren't easily available over here. They are very good value.

 

A magazine ad in Evo shows that Michelin considers its PS3 to slot in below it's earlier PS2 in the UK. The SS slots inbetween the older PS2 and the Sport Cup. This last tire is the semi racing street tire with shallow tread depth and fewer grooves fitted to some Porsche models. The SS is intended to be the highest performance Michelin you can buy and still stay on a streaming wet road.