95 Z28. The speedomter is off about 20mph. Meter says its going 70mph for example, but actual speed is about 50mph. Its been that way since I bought the car. Car had a lot of upgrades on it when I bought it. I only know of a few...ram air, headers, exhaust system, new MAF, new cam. 17in wheels. Auto trans.

My guess it that this car has something other than stock gears? 3.73's maybe?

 

That would be the most logical guess. You'll need to find out what rear gears you have, then your computer can be recalibrated accordingly.

 

You can figure it out pretty easily, first look on your door for the RPO code. Find one of these: GU2 (2.73) or GU5 (3.23). Then figure out the percent difference in speed from what gears you had stock, to what you have now, and you can figure out which gears you've got.

For example, my Z has 3.23s. At 55 I'm doing 55, but if I swapped to a 3.73, it's about a 13.5% drop in speed (to reflect the 13.5% higher gearing (numerically)) resulting in an actual speed of 47.6 mph.

And just because I didn't trust my own math, I went and figured it out with a rear gear/rpm calculator. You'll find I actually was right. :P lol