with gratitude I have a 95 z28 m6. I have the dilemma of the fact that my rear gears are 3.42s(I just checked 12 pinion and 41 ring). My speedo is accurate(checked with a gps) and as far as I know my tachometer reads accurately. However, I have run all of the rpm calculators and they say I have 4.10s. At 2000 rpm in 4th gear (1:1) with a 26 inch tire I am at 39 mph. I do not know where the missing link is other than it is driving me nuts and I cant think straight. does anyone have any ideas?

 

Your tach must be off. At that speed, if the tach was off by less than 200RPM, that would account for your discrepancy. If you want to put it to rest, get your car hooked up to a scanner (or a laptop with datalogger and cable) and verify the (in)accuracy of the tach - the PCM actually sees an accurate RPM, whereas the dash is (comparatively) approximate. I know my tach is out by at least 100RPM.

 

Sounds like a good idea. I can ease my mind with that. it also supports my need for 4.10s than!! lol I had posted on another forum and so far this has to be the best answer that I have recieved. over at ls1tech.com I am being harassed by guys saying that my stock tire is a 24 inch tall and that that is why my speedo is off

 

The stock 245/50-16 is 25.6" tall.

 

Stock tac's are just not the precision pieces. They can be off several hundred RPM.