Yes you're absolutely right, thats why it's so important to use a tire size calculator if moving between diameters. I have 17" rims on my car and the winter tires are 16" rims but I was able to match the tire sizes with a calculator so that my speedometer is 99.8% accurate as compared to the 17" wheels.

This one works well enough.

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

Jayson