gu8a,

Go to Tyre Size Calculator and click on the "Tire Size Comparison" tab on the right. You can then enter different tyre sizes and it calculates the difference in diameter etc and even tells you the difference in speeds. The size you suggest (although I think it will look really odd!) is only 0.3mph difference at 80mph so they are very close in circumference.

Ala_xfire, I assume you meant a 'lower' aspect ratio on a wider tyre to retain the same rolling radius, as a 'larger' one increases the tyre circumference/rolling radius and (obviously) the overall diameter of the wheel/tyre combo.

As a matter of interest, I retain the standard rims on mine and went up from 255/35/19 to 275/35/19 on the rear (to give added protection to the rims, having just re-furbished them). My speedo no longer over-reads, now being absolutely dead on with Sat Nav at all speeds between 30mph and 70+. I went up to 235/40/18 on the fronts for the same reason without any rubbing problems.

Hope some of this is of some use to someone!