Hi,
I'm getting 18's for my 220, , threre are 2 tyre sizes available to me, 215/35/18 or 225/40/18. Can anynoe advise me as to which are best for the new IS, I don't want to affect the speedo if possible and don't like an overly harsh ride

If you don't want a harsh ride then go for the 40 profile.

Go for the 225/40/18 ... the other one you mentioned is an odd size!

What type comes on the sport? their 18"s aren't they?

On the 250 sport they are ..

18" alloy wheels: Tyre size (front) 225/40R18 + (rear) 255/40R18

[url="http://www.miata.net/garage/tirecalc.html"]http://www.miata.net/garage/tirecalc.html[/url]
try this

[quote name='Brendan' post='395346' date='Nov 14 2006, 03:02 PM']Hi,
I'm getting 18's for my 220, , threre are 2 tyre sizes available to me, 215/35/18 or 225/40/18. Can anynoe advise me as to which are best for the new IS, I don't want to affect the speedo if possible and don't like an overly harsh ride[/quote]

Hi,

215/35x18 & 225/40x18 are two totally different sizes.

As a replacement for 225/45x17 (front) you should go for 225/40x18
and 245/40x18 for rear.

You can try this tool, even tough it is in french it is pretty handy :

[url="http://www.mtk.tm.fr/pneu_jante/jantes.php"]http://www.mtk.tm.fr/pneu_jante/jantes.php[/url]