Hello I want to buy new tire for my jaguar xj8 x308
Now I have 245/45r18 it looks and work perfectly fine for the front but I don't like the tail looks very low and 245 tire just doesn't look perfect for the car the suspension in good condition and all rear bushing are fine can I run 255/50r18 for the rear without problem ? I have asteroid wheels

 

The XKR uses 255/45-18 for the rear and 245/45-18 for the front.
The larger diameter must always be on the rear axle.
The rear wheels/tires must spin at the same rate (or slower) than the front or the TRACTION CONTROL will be enabled to slow the rear.(the ABS module thinks the rear wheels are 'spinning' and losing traction)

One of my XKRs has 275/45-18 on the rear and 245/45-18 on the front. (previous owner fitted)
No traction issues with this setup.

The sedans use the same ABS modules as the XKs so I see NO ISSUE with what you want to do.

 

Looks like it's complicated I will go with 255/45r18 and will try maybe coil spring buffer

 

Learn something new every day. I've assumed since the factory sire size is a square setup, that you would need to use the same circumference tires at all 4 corners or the TCS system would go nuts.

SilverJag, make sure you understand your speedometer will be off as the tire size you're proposing will be a full inch taller than the factory size. Speedo will read a few mph low at highway speeds is all.

Is the issue you're trying to resolve is the rear end looks saggy?

 

Not saggy but it's just low I know this is how the car designed but I wanted to make it higher by inch or two I'm not interested to run 19 in wheels
Any idea about rubber spring buffer?

 

Check the rear spring isolators and upper damper mounts.(they deteriorate and the foam falls off)

I have replaced all the sedan rear isolators/mounts but I need to replace all my XKRs and XK8 convertible rear suspension damper/spring mounts.

 

Spring spacers might be cost effective.