Can anyone tell me what gear i may have

[SnakeEater]

I have a 98 z28 the rear is out of an 02. The guy I got the car from said it has 3.73 but I don't think it could possibly be. When I'm in 6th running 70 my tach is at 2 grand. 17 in stock tires. I'm getting ready to pull the cover off but wanted to know if there is a way to tell by the rpms

[35th-ANV-SS]

Yes. There are a few ways to tell. You can use an online calculator to determine what RPM you should be at with what speed given your tires. Typically 02 cars would either come with a 3.42 (M6) or 2.73 (A4).

Based on your RPM, it sounds like the previous owner put in a 4.10 or 4.11 gear though because your RPM at 70MPH is about where mine is, and I have a 4.11.

You can also check without pulling the cover by jacking the rear end up (securely), rotating the rear tire one full revolution and counting how many times your drive shaft turns. If you turn the wheel (1) full revolution and the drive-shaft turns 4X, you have a 4.10 gear. If it turns 3.5 times, you have a 3.42 gear...and so on. It's easy to determine how many times the drive shaft turns by marking it with something (paint stick, tape, etc) so you can physically see when it turns.

[Cutlass]

yes, there's actually a sticky for that in this section: Gear/MPH calculator
I just ran the numbers and yes, it has to be a 4.11 or even a 4.56 comes close to that also

[wileyCoyote]

my car runs like 1700 rpm @ 70mph,..so you've probably go the higher ratio.