If your Speedo was reading 60 mph with 15 inch Wheels and Tyres, then assuming your Speedo was accurate, you would be doing 60 mph.

But if you put on 16 inch Wheels and Tyres and your Speedo was reading 60 mph.
would you actually still be doing 60 mph or would it in reality be more or less than 60 mph.

Would 16 inch Wheels and Tyres give you better or worse acceleration.

 

Wheels by themselves make no difference. It's the wheel tire combination that does. You have to know the rolling diameter of both combinations to answer this question.
There is a section devoted to this in Kirby Palm's "Book". There is a free download of it in the stickies in this forum.

 

To affect acceleration you need to change the tire's outer diameter, which may not be necessarily the case, read on.

Keep in mind that the diameter part of the tire size has everything to do with the part that matches the wheel, so a 15" tire is 15" at the edge that will wrap around the rim and only mildly affecting its outer diameter. Most of what affects the tire's outer diameter has to do with the rest of the tire's dimensions, such as basic tire size, as in 225 or 235, or 55 and 60, which represents the % that the side is from the width, which is the reason why a 55 tire would show a lower profile than a 60 tire because the side is only 55% of the width and so
forth. But I know that there are tables that include the final outer diameter for a given tire spec. The final conclusion would be that, no, a 16" tire is not necessarily one inch bigger than a 15" tire in outer diameter size.

When I got rid of my Lattice wheels, I actually went up 2" in diameter to the 17" Pirelli Pzero's.
But the fact that the new tire was a 245/50R17 low profile tire helped to fit it OK, albeit a little bit close, but then again, I was going up whole two inches. As a consequence, my odometer is about 5% slow.

Cheers,

 

I think I'll stick with the 15 inch even though they can be quite expensive.

 

OB
When I changed my 15 inch starfish to 16 inch "modern starfish" I measured with string the precise circumference of the two with the tyres mounted and inflated. I had OEM Pirellis on both, from memory the 215x70 15s (but whatever the handbook said anyway) and replaced them on the 16 inch rims with 225x60 16s (again from memory but ditto).
The circumferences of the two were absolutely identical to an accuracy of at least 1/2 an inch, maybe better. As far as I could measure there was no difference at all.
Greg

 

I have found this site helpful:

Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

 

My son says numbers are not my strong suite. I'll not admit it, but.....


Indeed, it is the circumference that matters. As the diameter is a function as is radius, these can all be related.


But, all must include the tire as well as the wheel.


So, let's say the original 15" wheels shod with 215,x 70 x 15 rubber have a circumference of 31 inches. Now, the replacement 235 x 60 x 16 have a circumference of 34 inches.


An increase of 3". 3/31 will be the percentage of increase. Times that to the speedo MPH or KMH and get the new number.


Or merely Google up tables that do it for you!!!


Not sure about now, but it was not long ago that most Speedos were optimistic.


Carl

 

OB,

Others have described the relationship between tyre wheel and rolling circumference.

The most common XJS sizes and the total circumference in mm is as follows:

205/70 15 - 2098.58mm
215/70 15 - 2142.57mm
235/60 15 - 2082.88mm
225/60 16 - 2124.97mm
225/55 16 - 2054.29mm

This naturally doesn't allow for sidewall deflection etc, but it gives you a rough idea of the comparisons.

So, to understand the impact, look at the original size tyre for your car and compare against the upgrade wheel and tyre. If the new figure is lower, the speedo will over-read ie you'll be going slower than indicated. If the figure is higher, you'll be going quicker than you think.

Hope that helps

Paul

 

Yup, it doesn't surprise me at all.

Cheers,

 

This is my Favourite answer so far but for the time being I'm staying with the original 15 inch Tyres even though they might cost a bit more.

 

Hi Greg

The only downside from what I can see, is the actual cost of the 'New Style' Alloys, which would outweigh the cost of New 15 inch Tyres.

What make are the Alloys and are they easily available, as I would like to get some for my XJS Celebration Convertible.

As they look very much like the ones originally fitted.

My Car has the Flat Disc Type but I think the Starfish look better.

 

Many Thanks Carl

That's very useful to know.

 

Absolutely Excellent!

Many Thanks for taking the trouble to post that.

 

I went through this last year. Here in Aus you must use tyres with the same or better speed rating. The 215-65-15 that were standard here were R speed rated and almost unobtainium and only Michelin and Pirelli at $400 a tyre. I found a set of 16x8 BBS off an XJ40 and had the front modified then fitted with 245-50 16 tyres for less than the new 15".

 

Here, try this:
https://tiresize.com/tires/Jaguar/XJS/1992/Base-Model/

 

OB
The ones I have on my car (same as the photo in my post above) are OEM Jaguar rims as fitted to the facelift cars. They quite often come up on UK Ebay and breakers have them, and are often not too dear at all. The five spoke design is inherently far stronger than the lattices, which can develop fatigue cracks which are very hard to detect and can lead to failure.
Greg

 

OB,

As Greg says, those alloys are known as 5-spokes. There are 2 different types. The original ones were fitted to XJS from 93 (and also XJ40). They are painted and have a small groove across each spoke near the rim. The second series came out in April 94 and have no grudge. They have a diamond-cut spoke face with painted inner surfaces. I have these on my car. The same wheel was then chromed and offered as an option on the CeIebration models. In the UK, the chromed are rarest, followed by the diamond-cut. The grooved 93 wheels are the most available on the second-hand market.

Paul

 

Technically you can fit any rim size (diameter) and still get the same speedo reading. There is a reason why there are hundreds of tyre sizes

For example... A rather well remembered thing: my former Volvo C30 came from factory on 18" rims with 215/45 R18 tires. The 215/45 R18 tires required a software patch. With 225/40 R18 it wasn't required. With 225/35 R19 it wasn't required either. The overall diameter of the 225/35 R19 tires was about the same as 195/65 R15 tires.

You only ever want the correct tyre size. There are tonnes of calculators for this. If you know your current size, a bit of mathes will help...:

(((Diameter x 25.4)/2) + 1 x (%age of tread width)) x pi x 2

So, in my XJ-S case...:

(((15 x 25.4)/2) + 1 x (65% of 225 mm)) x pi x 2

(((381)/2) + 1 x (135)) x pi x 2

(190.5 + 135) x pi x 2

325.5 x pi x 2

2045,18 mm

So the tyre has a circumference of 2.045 m.

I would need to find a tyre on a 16" rim with the same circumference. So that would mean looking at say a 225/50 R16 or a 235/40 R16 if you want some wider tires.

 

Hi Daim

Thanks!

That's a very useful piece of (in a nutshell) info, so if I decide to go for 16 inch then at least I'll have a much better idea of what I'm looking for.

 

Hi Greg

Thanks!

I'm a bit of a 'Lattice Lover' but in any event I will keep a look out for some 5 Spoke Wheels, as I need some for my 'Facelift' Car.