If you run a 225/40/18 tyre on an 8J rim, will the rolling circumference be different to running the same size tyre on a 8.5J rim, due to the fact the tyre is more stretched on the wider rim?

Or am I talking complete b*ll*cks :S

In theory it would have a smaller rolling circumferance due to slightly smaller wall height but I suspect this will be such a small change that it will not be noticeable (probably no more than the change from full tread to minimum tread).

I would say the difference is SO SMALL it's not even worth thinking about it.

However, go much wider than 8.5J and you need to start thinking about offsets and fouling suspension/arches, etc.

Have a play on this!

http://www.rimsntires.com/rt_specs.jsp

Have a play on this!

http://www.rimsntires.com/rt_specs.jsp

That's brilliant Mike, thanks