if your changing tire sizes, this will tell you the speed difference.

input your current size, then your proposed new size.

bookmark the link

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

 

I found those calculations to be a touch on the high side. the numberical height of my 315's is 34.4 inches, however the actual value I had to use to make my speedo read dead on was 33.95 inches

 

did you select

STANDARD or METRIC ?

 

doesnt matter. . .they are useing the true math of figureing rev's per mile on an unloaded tire which is correct when weight on the tire is neglable. . .when a tire is loaded it flattens out a bit and is a little shorter.

 

That is the same site that Igave the dodge mechanic when I had to recalibrate my speedo. It worked great, but just as Drew mentioned, it may be a TAD off.I think it reads about 1 mph too fast now, but is so minor you can't rellytell. I'm going against my Garmin. I'll look at my speedo and my Garmin and both will read let's say 20 mph, but once I get up to 55 on my Garmin my speedo will read about 1mph off.

No biggie ...