Tony Trans Am

What rpm does your c4 show at 80 mph? My C4 feels so under geared.

Zrxmax

Here is a site you can sort out gear ratios and figure out what to do next.

My stock 88 with an auto and 2.59 rear gear was 80 mph at 2000 rpm. My 91 Z with 3.07s is approx 90 mph at 2000 rpm.

What gears do you have now?

http://www.angelfire.com/fl/procrastination/rear.html

WVZR-1

Your '90 tach is probably not anywhere near correct. You want to put a scanner on the ALDL and confirm tach operation before you assume anything. If you've C68 AC you can use the AC control panel for some diagnostics, one of which is RPM but the best procedure is just put the scanner to the ALDL.