I am trying to determine the axle ratio in my '84 Vette.  It has an auto trans and the housing says its the standard 2.73 ratio that came in the the automatics, tire size is stock.  Mathematically I come up with 3.54 ratio, but that seems a little low.  I am turning 2800rpm at 60mph in third gear (not overdrive); speed was checked using GPS rather than relying on the speedo with I know is off.  This car is a lot of fun to drive, and it jumps out of the hole.  I guess what I'm looking for is thoughts from others give the rpm and mph.  I appreciate the input. - Thanks.

Jack up both rear wheels and mark the drive shaft at the "U" joint, near the rear housing.

 The method by blacksharkL82 is the best way if youcan get under the car. MyCor-Vette Specs book lists 2 ratios. The3.07:1 was probably the one used in the auto, 3.31:1 was listed but doesn't say auto or standard. The lower was probably for the standard. There was also a 4. There was also a 4.10 available as an option. An option G92 PerformanceRatio available but I've never been able to pin down exactly what that ratio was. I expect the 3.07 came in your car.

Thanks guys.  I actually raised the car and spun the tires and to my surprise my calculations are correct.  One tire rotation gives 3.5 drive shaft revolutions.  I checked three times, once spinning the tire in the opposite direction to make sure I was not experiencing slop in the rearend.  I am not the only owner of the car, so I don't know if the gear was changed or if someone swapped in a later rearend.

you might have a later gear. My '96 LT4 has a 3.45:1 Dana 44. That's very close to what you're coming up with. The auto's came with a Dana 36 and the higher ratio. Dave