Anyone know off the top of their head the rolling circumference of say a 205-50-15 compared to say a 195-70-13 ?

http://www.miata.net/garage/tirecalc.html

Carbible suggests 1840.97 for the 15 against 1895.01 for the 13.

That's 2.94% bigger!

http://www.carbibles.com/tyre_bible_pg2.html

mIKE

Thanks guys, great link.

Try this, very quick comparasons between tyre sizes and road speeds

http://www.rochfordtyres.co.uk/tyrecalc.asp

Hi if you are after accurate info best to measure an actual fitted tyre.

Almost all of the info thats out there is out by up to 40 mm per revolution in some cases. Also different manufacturers of tyre differ greatly. Means you can end up in a right mess if your looking for a definitive way to jump on the gearing.

Cheers Matt