A 255 tyre on a 10" rim is a little over-sretched. And you are correct, in a static state, the absolute tyre circumference is a constant. But mounted on a 2-tonne car isnt exactly a static scenario. Under load, I'm guessing the over-stretched rears will have a larger contact patch than the fronts, and therefore be running slightly faster.

But you are probably also correct, the difference isn't likely to be much.

Having said all that, wide tyres like these arent normal for all terrain tyres. Narrower tyres dig into soft ground/mud/snow better than wider tyres.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)