I need a little help from some of you that are a lot more knowledgeable about tires than I am. I have a 2000 XK8 that needs new tires. The previous owner didn't like the XK8 wheels so he changed the wheels to those on an XKR. The wheels measure the same rim diameter and the wheels are both 18 inch wheels. But the tires are different on the front compared to the rear. The front numbers are 245/45 ZR18 96Y. The rear ones are 255/45 ZR18 99Y. They are both Micheland (sp) tires. What is the difference between the front and rear? And why are they different? I don't like having a different tire on the front versus the rear. Can I replace the tires with new ones of the same size and could that be the front ones?

Thanks guys for any help you can give me.

 

I know my 03 XK8 has wider tires in the back then the front. With out looking at mine those sizes look right though I don't know the diff on that 99Y and 96Y part.

 

The wheels AND tyres are different - same diameter, but wider on the back for better traction.
Totally normal - most XK8s and XKRs that don't have 17 inch wheels are like that.
You could put narrower wheels on the rear, but not wider wheels on the front, to get the same all round, but that would be an expensive way to "downgrade" the handling of your car.

 

EZDriver according to the Jaguar specification those are the correct size/rating of tires.

 

The staggered wheel/tire sizes are normal except for the 17" wheels.

The rears are not just a larger tire, the rim itself is wider.

 

Thanks guys. I was pretty sure they were right for the XKR but thought the XK8 was the same all around. Since I am a conservative driver having them the same would all frequent rotation and longer tire wear. Not sure if the rims are identical and would allow the same all around. And this car did have 17 inch wheels before the previous owner wanted the wheels on the XKR. The 18 inch I now have cause a 5% error in the odometer reading, slow.

 

Someone would probably swap your 9" wide rears for 8" wide fronts of the same style.

If I had 18" wheels, I would, but mine are 17"s. I would love to have 9's all around. Rear 9" wheels should fit in front/rear with 255/40's with maybe a wheel adapter), which would also lower the speedo error you are seeing (they are 26" tall vs 27" tall)

The going rate for good chrome 17"X8" rims in Dallas is about $350/set of 4. A staggered set of 18" would likely sell for slightly more.

 

The speedo difference is negligible due to the larger wheel diameter being compensated by the lower profile tyre. A 245/50R17 does 757 revolutions per mile. The 255/45R18 does 746 revs per mile. So the speedo difference is 1.47% (the 18s read slower by 0.9 at 60mph).

 

I'm pretty good a calibration of sensors after a career of flight testing high performance aircraft. There is a definite 5% error which is no problem as such.

 

My hobby is open road racing which is basically a 140 mph 1 or 2 leg TSD rally so speedo/odo calibration factors are like a mental disorder for me. If a speedo reads more than 1.5 mph off on the highway, I'll change tire size to make it right.


On our race car, the speedo takes a reading twice every tire rotation and we put 200 miles on it just developing tire growth curves to compensate for tire growth at various speeds. At the 2012 Big Bend Open Road race, our Odo calibration was only off 13 feet after 118 miles averaging 140 mph.


Automotive OEM's don't seem as concerned with speedo/odo error. I have always suspected that GM puts smaller diameter tires on some vehicles because they are cheaper and the car goes out of warranty faster.


With 3.06 rear gears, shorter tires won't hurt an XK8 in any way... an XKR sure, but not an XK8.

 

Not doubting your abilities, but something seems wrong here.


The overall diameter of the wheel + tyre should remain pretty much the same irrespective of which you have got.
So - an 8J x 17 wheel takes a 245/50/17 tyre, and a 8J x 18 wheel takes 245/45/18 tyre.
The "rolling circumference" will be virtually identical in both cases.
Obviously, the width of the wheel tyre (8J x 18 or 9J x 18) won't have any effect on the speedometer reading.
If you are 5% out - (not sure how you are certain of that - GPS comparison ?) - then, in my humble opinion, there's something wrong with the software calibration on the speedometer.

 

If you want a cheap set of good tires with those sizes, Tirerack has a set of 4 Sumitomos for about $420 + shipping with no tax.


The last set of tires I ordered from Tirerack on a Sunday night showed up Monday afternoon on my porch, and you are even closer to the Shreveport warehouse than I am.

 

I agree with you it should be the same. And maybe the error isn't due to tire size. But that is all beside the point.

What I am really need to know are the rims also different front to rear or just the tire size?

 

Rims AND tyres. The difference isn't about diameter or circumference, it's about width - wider rims and tyres at the rear for better traction.


Not totally sure of my facts here, but I don't think you could put a 255 tyre on to an 8J rim or a 245 tyre on a 9J rim.


You could fit 8J rims (245 tyres) all round, but you could not put 9J (255) rims on the front as they would probably foul the wheelarch on full steering lock.


What sort of spare wheel have you got - if it is a "spacesaver" - that's so you have a wheel / tyre combination that will fit on all four corners with no problem.


Just to add to the complications - tyres can be asymmetrical and/or directional, which limits interchangeability in terms of front / rear & left / right.

 

XKR's came with 9" wide wheels with 255 wide tires on the front from the factory (20" option), so that should work fine. Even better with 255/40's which are shorter than the factory 20" setups.

255/40R18 or 255/45's's will fit fine on 8" or 9" rims, so if you wanted one tire size, either of these would work. You still couldn't rotate them without looking dumb, but it should still handle OK and either would fit front and rear.

I've seen no evidence an XK8 really needs larger tires in back and probably handles better with the same size tires all around.

 

Thanks guys for all the help.

I've got to check and confirm what size the rims actually are since this was not a factory installation but what the previous owner did to get better looking rims.

 

The rear rims are one inch wider that the front rims. So I am stuck with no front to back rotation. I'll just stick with the existing tire sizes. Maybe go with the Sumitomos tires. My tire guy has recommended them. Any comments would be appreciated as always.

 

Where can you run 140 mph on open roads?

Or is this on closed roads?

Have you ever considered a Halda, or whatever it's
successor might be? (Have one kicking around here
somewhere complete with front wheel cable drive).

With an ABS sensor in the front hubs, it would be
pretty easy to build a dedicated time/distance/speed
computer with digital display for under $100.

 

I have had 255s on fronts and rears for a good wile now,never had any issues

 

They just call it open road racing because it's done on the highway. The roads are definitely closed.

We've won the 135 and 140mph classes at the Big Bend Open Road Race where they let you go 168mph max.



We had to creep up on 168mph for this picture (with 4 GPS and the Rally computer all saying 168 before we went into the speed trap), because if we went 169 we'd have been DQ'd from the race. You get a "Speed Certificate" with your official speed through the trap, so it was always our goal to get one that said "168 mph".

We also have done the Sandhills Open Road Challenge in Nebraska for the last 15 years, which is a blast.

The Odometer pickup is on the driver's front wheel to measure centerline distance/eliminate wheel spin errors.

I would love to be able to build my own rally computer. The current computers only are only good to 1/20 of a second and the military GPS guys I compete against have boxes that read to the .001 of second.