Hi,

I need a pair of back tyres for my '98 XK8.

The tyre wall reads: 245/45 2R 18 (100Y).

When I use the 'enter your reg no' tool on blackcircles and elsewhere it says that for 18" wheels, the tyres should be 245/40s.

I'm now not sure what to do when replacing the tyres - should I get some 45s or 40s?

Also, the speed rating for Y is 186mph. Is this overkill, seeing as I never do more than 90mph ?

 

hi,you should have 245/45/18 front 255/45/18 rear as standard, i have 255/40/18s front and rear bridgstone runflats also y rated and there absolutely fine but if you want to stay standard then its the 255s rear but stay with the 40s if your happy with your fronts

 

Someone with more knowledge and expertise will probably give you a better answer than this but .............
Are your front and rear tyres the same width ? In my (1997) handbook and on my car, the front wheels are 8J x 18 and rear wheels are 9J x 18. You should be able to tell - is there a plastic trim round the rear wheel-arch which was a legal requirement I believe, to ensure that the wheels were not wider than the car. (17" wheels are the same size all round)
My handbook specifies 245/45 ZR 18 96Y directional for the front, and 255/45 ZR 18 99Y asymmetric for the rear.
I suspect that for "normal" drivers like us who rarely, if ever, :-) exceed the speed limit, asymmetric / directional / 160mph speed rating etc are all a bit academic EXCEPT that in the unfortunate circumstance of an accident, your insurance company might use "non compliance with manufacturers recommendation" to wriggle out of a claim.
Your call I guess.

 

I didn't think of looking in the handbook - I'll do that, thanks.

 

While it may seem that buying a tire (tyre) rated for 160+ is overkill for a car that is speed limited to 155, and never driven over 90 mph, there is more to that rating than speed.

To achieve that rating, the compound and makeup of the rubber is improved in grip, strength and durability at speed. You are getting a better tire. An added factor is the weight of these cars also require a sturdy set of tires. All this improves handling performance at normal speeds.

The only thing that will suffer on a speed rated tire vs. a non-speed rated is tread life. The stickier speed rated models are usually only guaranteed for ~45k tops.