how to tell what gears are in my car

i guess the guy i bought my car from put gears in it without recalibrating the speedo. It is a 1999 z28 the speedo reads 40Mph at 33mph
and 50mph at 42mph and 60mph at 49mph and on and on. stock is 3.42 so i am guessing it has 3.73 or 4.10's , if anyone knows how to tell what gears i have or if it is something else causing the error on the speed o please tell me

here is the math to do ur problem.
(rpm)/[(final drive ratio)*(mph)*(336)/(tire height)]

dammit math hurts my head

You could also check your RPO numbers and see that way.

yea but he says that the guy he bought the car from has changed the gears. Which also reminds me that he is gonna have to know how fast exactly he is going to do the math. So scratch that. check and see how many times ur drive shaft turns in one complete turn of the rear wheels.
3and 1/4 turns =3.23
3and 3/4 turns =3.73s and so on

Oh I didn't bother to read that part cause I'm a dumb ass but you can always open up the rear and change the fluid. That will be one way but Z28's ways will work just fine too.

or even better
call the guy you bought the car from and ask him what he put in it

then call up madz28.com and get the computer calibrated so you know how fast you are going and all that fun stuff