96Firebird

My friend bought a '99 GT a while back. Its been modded, headers, exhaust, other random stuff. But the speedometer is off, so I'm guessing he either has gears or someone messed up installing the reverse glow gauges. But his car also goes through gears pretty quick, so I'm gonna say he has gears. Is there an easy way to find out his gear ratio without tearing into the rear end? Say, he follows me going 55MPH and we get his RPM reading. Is there a formula or something I could use to figure this out? I'm guessing they are 3.73s, since 4.10s seem a little high. Thanks for any help.

ericdiver

lift the wheels and mark the drive shaft, turn the wheels and count how many times it turns.... you can get pretty close