How fast am I driving?
 

I have a 94 with 18" wheels and I wanted to know how accurate my spedometer is. What is the best way to figure this out? the best way I could think of was to drive down a known distance (1/4 mile) and compare that to the reading on the odometer. But our odometer is digital, and it only shows resolution to 1/10 of a mile. So i think this would be inaccurate.

Check out this Website: http://www.miata.net/garage/tirecalc.html

Compare the percentage difference you get to figure out how fast or how slow your spedo is now.