Wheel size(s) and tire size mix-up.
 

I have a 2001 XK8 that came with the 18" wheels. The tires should be 245/45/18 on the front, and 255/45/18 on the rear, but I just noticed I have 245's on all four wheels! 1. Do I or should I replace the rear 245's with the proper 255's? The tires are Continental Extreme Contact DWS tires with only 6,000 miles on them and they seem to be working fine. 2. Are all four wheels the exact same size? And if the wheels are two different sizes, how can I tell one from the other? I have no reason to believe that if there are two different sizes, the right wheels are on the right axles.

Turn the wheels all the way in both directions and check the inside of the front tires and the wheel well area for scuffing, scrapes, etc. With 255's spec'd on the rears, safe to assume they're supposed to be 9 inch width wheels. The 245's on the 9's should be ok.

I have a 2000 XK8 and the previous owner replaced the 17 inch wheels and tires with the same tires that are on the XKR which were 254/45 ZR18. As a result of the one inch larger diameter wheels the odometer and speedometer run almost exactly 5% low. When indicating 70mph I am actually doing just shor of 74.

Just a little tid bit of trivia.

EZDriver

Just stick a tape measure through the holes in the rim and measure the width of the rim. The wider ones should go on the rear. Having 245s on all four corners won't really make a difference, but you definitely want the wider rims on the rear. And you especially don't want both wide ones on one side of the car, or the matching rims on opposite corners.

Rev Sam,

Do you know for certain that the 18" wheels are all the same size? I have the same ones you have in your photo.

Mine are 19's. I think the fronts are 8 inches wide and the rears are 9. I don't think they made the Apollos in 18 inch diameter, but maybe I'm wrong. Just look at the size on the side of your tire. It won't hurt to have the 245 tires on all four rims, just make sure you have the 8 inch width rims on the front and the 9 inch width rims on the back.

All the factory wheels that are 18" or bigger are staggered width. Only the 17"s are the same front to back. I believe Sam's wheels are 19" If the wheels on your car were replaced, there is no way to tell without measuring or looking for the casting to see if they are all the same size (using 4 front or 4 rear wheels) or whether someone used the proper staggered widths and simply put tires sized for the front wheels on all four wheels.

I'VE FOUND THE ANSWER ABOUT WHEEL SIZE! Rev Sam, thanks for the tip about the tape measure. It easily and clearly indicated that two of the wheels on my XK8 are bigger than the other two. Fortunately, and without any insight from the idiots at my local tire store, the bigger rims are where they should be, on the rear axle. For the record, my 2001 XK8 has the optional 18" alloy wheels. The 2001 Dealership Sales brochure shows a photo of the optional rims and calls them the "Impeller" alloy wheel. The photo shows a rim with seven paired spokes. None of the other pictured wheels have seven spokes. The brochure describes them as only 18" in size and makes no mention of there actually being two different widths supplied for each car. HOWEVER, the factory supplied Vehicle Care manual that came with the car lists specs for the different wheels that were available. The problem is, there are no photos, and the names of the different models don't match up with the names used in the Dealer Sales brochure! The Vehicle Care manual indicates that for the 2001 XK series cars one could get: Revolver or Lamina models, which were both listed as being 8Jx17 rims; a 7-Flute model, which is 8Jx18 front and 9Jx18 rear; or lastly, the Double Five model, which are also 8Jx18 and 9Jx18. Being a trained investigator, I suspect that the "7-Flute" model is the same as the paired seven spoke "Impeller" model, especially since photos of the rims offered in the Dealer Sales Brochure show no other rims with seven spokes, paired or otherwise. Before I measured the rims with the tape measure, I closely examined the wheels as the car sat on the ground, hoping to see an 8J or a 9J mark or serial number. If there is such a mark, it must be inside the wheel under the tire, or on the back side of the rim. In any case, the tape measure quickly and easily showed me there were two different sizes on my car, and which was which. Reverend Sam, thanks.

It seems like the Impeller wheels replaced the Fluted (also known as Sport) wheels in 2001. They are 8Jx18 and 9Jx18 as you suspect. Here are the Fluted wheels on my 2000:
https://oi43.tinypic.com/2pqjmg2.jpg

I guess having incorrect tire sizes in the wrong position has no affect on the ABS sensor? Jack

I think if you had significantly different circumference on tires opposite each other, front or rear, there could be ABS trouble.

As the wheel size increases, the side wall width decreases. That is if you had 245/45 ZR17s and do a +1 conversion, you'd go to a 245/35 ZR18. A +2 would be 245/25 ZR19. Regardless, the full diameter of the wheel is approximately the same and therefore your speedometer should remain correct. However, if you replace a 245/45 ZR17 with a 245/45 ZR18 then your wheel diameter will be larger and your speedometer will not be accurate.

Mark