What are the widest rear tire sizes I can install on my 2000 XJR Asteroid 18x8s?

 

https://www.jaguarforums.com/forum/t...calculator.php

 

The tire shops are right in this case, the 255's fitted by Jaguar are already on the limit.
You will need wider rims, or staggered with a wider rim on the rear, to allow for wider tires.

 

These are recommended (approved) rim widths, by one of the tyre manufacturers, for various tyre sizes:

 

We have all stated recommended spec tire sizes because going any larger and you lose performance drive ability. Tires have changed regarding ability and reputable tire shops will have the latest info for applications. The x308 is a heavy steel body car (about 5k lbs) and when you go into a corner or brake you put a force load on the sidewall of the tires (both inside and outside of the rim). Which is why certain tires are recommended for certain cars, ...by putting on a wider wheel than the rim size, you are taking the chance of damaging the sidewall and risking wheel damage or perhaps an accident, not to forget to include balancing issues and vibrations.
My VDP(R) has 255/40 18 Michelin Super Sports on Penta rims. I highly recommend not going with any lower than a 40 sidewall, you'll get a stiff, rough ride and it doesn't look good in the wheel well...too much space.