HEy I have heard a lot of stuff on changing wheel diameters causing speedo inaccuracy. LEt me know about my recent upgrade from 15"s to 17"s. Is the speedo affected and how so, faster in reality, slower, etc. tanx

 

it doesnt change much if you use the right tire. maybe 2 or 3 mph lower. the correct tire size is 205/40. check out the miata site it has a calculator @ http://www.miata.net/garage/tirecalc.html

 

Get on the highway, find a long stretch of flat road, set your cruise control to some convenient speed, then use a stopwatch to time yourself between mile markers. Do it several times if you want to and average the result. Then divide 3600 by the number of seconds it took you to cover a measured mile. This will give your actual mph. Or if you know of some place that has one of those police signs that displays your speed, go drive in front of one of those when there isn't much traffic around.