well i got my speedometer situation fixed, however, there's still one tiny problem. the speedometer is off. last night, i told my friend to pace me and he was going 55 and my speedometer said 45. i've got my rx7 wheels on there still. is the stock tire size 205/60/16? and is the speed measured by the transmission or how fast the tires are moving? thanks...arjay

 

Speed is calculated based on the output shaft of the transmission. Incorrectly matched speedo gears to the rear end, incorrect tire size, and lack of lubrication in the speedometer will throw off the output readings.

Your tires are a lot bigger than stock; this is your problem. Stock should be a 50 Ratio for 205's on 16's, so 205-50R16.

The only benefit your are gaining with these larger tires is better fuel economy through lower revs at highway speeds. Your acceleration will also be suffering from the larger tire size.

 

Yeah, I have the opposite problem but its due to my rims/tires. I am running 17" rims with a 40 series tire and my speedo is off, it says 120km/h when I am only doing 110km/h (according to my friends car pacing me).