www.jec.org.uk

Just got a puncture on a rear tyre and as I got the car with a cheap nasty set just on the rear I was thinking of putting a pair of decent 245x45x18s I have already instead of the current 255.

Are there any issues with doing this?

I don't have all the various years' literature on XKR's but all that I have give 245/45 (F) & 255/45 (R) for 18" wheels. If you change from this you will get a different rolling circumference which will alter the accuracy of your speedometer and trip meters. You will also need to check that the proposed tyres have the correct Speed & Load Ratings.

In addition to this, your change may result in your Insurance Cover being null & void if you fail to inform your insurers.

AllanJ

Thanks very much for the insight.

If I have my maths right the 245x45 will be 1cm thinner than the 255 therefore profile 0.45cm thinner; so circumference 2x0.45xpi shorter. That's just over 1 inch so just over 1% different to that with the 255.

I guess as my speedo is about 5% out from the GPS anyway I won't notice the difference. Will make sure to tell the insurer.

I like your Maths but you've forgotten there's another dimensional variable:- that the tyre profile is also governed by the wheel rim width. On the XK range (at 18" dia.) the 245 tyre is put on a 8" wide rim and the 255 tyre on a 9" wide rim.

AllanJ

Update for anyone interested - now have the 245's all round and have done a couple of journeys. Am very happy with them, Dunlop SP Sport 01 noticeable quieter than the cheap and nasty 'Sunny's that came on the car. Speedo is (guessing) maybe 1mph less accurate vs the sat nav at 70/80 than before (it was already about 4mph off and now about 5mph). If you look closely the fit doesn't look quite as good as the 255s.

I will probably run these until the fronts need changing which should come up next, rotate, and then put 255s back on.

Also FYI my I did call my insurer and they weren't remotely interested.