These are very usefull when you getting different wheels and need to know what size tire to run to keep your speedo just right.

miata.net


rims-n-tires

personally I like the one at 1010tires.com, i've found it to be accurate within .01%

I've had good luck with the miata.net one. I just recently found the rim-n-tires one, I like how they don't allow it to do stretched tires.

http://www.dakota-truck.net/TIRECALC/tirecalc.html ........ this one is pretty good too, compares the four dimensions of both sizes side by side, breaks down speedometer differences in 5MPH increments from 5 - 200 mph, also shows you what the effective gear ratio change is with different sized sneaks...