I have a friend who has a 2001 xk8. He asked me if he ould replace is stock 255/45zr18 tires with 255/55zr18. I have no idea why hwe wants to do that. I made some ruff measurements and it appears that they should fit - but it'll be close. Plus , unlike on his 2000 corvette, he will not be able to correct his speedometer readings and the stance of the car will be somewhat "off".
Any ideas ??

 

The car might lose a little acceleration due the the increase in tire size but 'might' get a slight increase in MPG?
Lots of people want to spin the diff toward acceleration rather than top-end.

 

Is that 2000 xj8 for sale?

 

I'll weigh in on this. At the moment I'm upgrading from oem wheels and tires, so careful evaluations were a necessity. Vehicle: 2001 XK8 conv., 73k miles. Wheels and tires are oem as delivered: 8.0" F and 9.0" R.@ 18". Continental tires at factory size and ZR rated.

At the rear, I'd suggest the interference resulting from this increased overall diameter will likely [high probabilty] occur at the Suspension Locating arm. It is longitudinal, formed steel which 'trails back'.
Where you ask? View driver side profile, rear portions of vehicle, think hands of a clock face...The interference is at 9 o'clock, on the tread backside, at the tire shoulder.

Got to tirerack.com, select to use 'Shop by Tire Size', then after that, go to the brand, then to Specs. You will want to, indeed do not exceed 27.5" Maximum diameter. My guess is their default sizes will encourage 26.7" diameter max to maintain clearance and speedo/ odo utility.
That 0.55 aspect ratio in any fitment will make the ride 'soften' and deteriorate handling. All four would need to be 0.55 incidentally.
Summary is retain the 18" at 0.45...or, go to 17" to improve the ride. There the default ratio is a softer 0.50 ratio. Fivefive aspect ratio is not an XK8 formula.

 

255 45 fits fine on asteroids pentas and all the 8 inch wheels

i’d step up to 275 40 rears if you have 9 in the back