I am trying to figure out what gears I have.

The rear end was replaced in the car so I can't look at the codes.

I used the only method I knew by jacking the car up, spinning the wheel once, and counting how many times the drive shaft spun.
This lead me to believe I have 3.23s.
Which is what I was told was in the car.

I then used the gear chart rpms/mph and it says I have 3.73s.

My car is an auto with 275/40/17 tires.
At 70mph I'm at 2500rpms.

Anyone know why these two methods give different results...

 

That's a really difficult way to do it...

Best way to do it would be to pull the rear end cover off.. Then count the teeth on the pinion and then count the teeth on the ring gear... THen you divide the teeth on the ring gear by the teeth on the pinion and you have your answer..

An example for 4.10's would be 41 teeth on the pinion / by 10 teeth on the ring gear = 4.10

Then ring gear is the LARGER one. The pinion will have less teeth.

 

I was hoping there was a guaranteed way to tell gear ratio without pulling the cover.

I see most people use the rpm/mph chart.

Is there anything that would throw off the rpm/mph chart?
Torque converter not locking for example...

 

Yea there is a guarnteed way to tell.. Pull the cover off and start counting

 

Take your car out on the highway, drop it into the 3rd, and run it at 70mph. If the RPM's are around 27-2800 you have 2.73. If they are above 3000 most likely 3.23. It worked on my car... hope this helps.

Mike

 

i would say youd be closer to the 3.73s but thats a guess because i have 3.42 and im not that high of an rpm at 70mph

 

according to fbody gears if you are at 70mph at about 2400-2500 in 4th gear, then you have a 3.73 rear gear.