If you don't know if your gear is stock, you can compare your RPM to MPH when you are in 4th gear (1:1). For stock tires, you should get the following MPH at 2000 RPM
2.73:1 54 MPH
3.08:1 48 MPH
3.27:1 45 MPH
The basic formula for this is:
MPH = RPM *60*3.14159*D/G/12/5280
where D is your tire diameter in inches and G is your gear ratio. Tach and speedometer accuracy will vary!
one last old school way, jack the back end up, put the trans in nuetral, mark the tire and drive shaft with chaulk, and precisely count the revolutions of the drive shaft to make one full revolution of the wheel. this only works with posi rear ends. if it takes two and three quarter revs of the driveshaft to rotate the wheel it is a 2.73-1 and so forth.