If you want to find out if a different tyre size will affect your car before physically changing them, here is a quick resize calculator I stumbled on:

CLICK

Thanks for posting that. Just sneaked a quick look and my new 13" rims/tyres compared to the old 14" on the donor means I can use the donor speedo without re-calibrating (only 3.36% difference) - great!

HERE is another calculator, HTH

Nice!

HERE - Click this is probably the nicest tire resizer I have seen. You will NOT be disappointed! Every dimension you could want. Including % difference from one size to the other and more.

Enjoy!