Tell what gear ratio I have? I looked at the window sticker and it says that I have the locking diff but no ratio is stated there. I know at 70mph the rpm is at 1750. Its a six speed auto 2011 2500 4x4

 

My guess is you have 3.73 gears. My 07 runs about the same rpm for that speed

 

my cousin showed me a trick in his mustang that a performance dealer told him to do to see what rearend he had...since that car is a 5spd they said to drop it in 4th, run it up to 73 mph and see what the rpm's say...it was settin dead on 3, which would make it a 3.73...i tried it in my truck, the one in my sig, it bein 6spd, i dropped it in 5th ran it up to 73, and there it was...sittin on 3 on the tach....hope this helps ya....never knew it myself til then

 

What size tired do you have

 

Sorry I should have posted tire size. They are 265-75-17. Thanks for the replies

 

best way to know for sure is to lift the rear end, turn the tires 1 full turn, and count the revolutions of the driveshaft

 

If you have your list of options and you have DMEP that means 3.73 gear ratio.EJT=cummins DG7=6-speed auto.Im not sure of the codes for other ratios.I know that GM has all codes on a sticker on the glove compartment door but not sure if Dodge does the same.

 

should have a tag on one of the bolts on your diff cover also if it hasnt been apart and left off

 

Thanks for all the replies. 3.73 it is.

 

This wouldnt work with every vehicle though, because every vehicle has different size tires and different trannys (thus different gears).

here's a formula for the OP.
rpm=(mph x gear ratio x 336) / tire diameter

Final drive on a 68RE is 0.62 btw.
RPM=(70x0.62x3.73x336)/31
RPM= 1755

so at 70mph with 3.73's and 31" tires should be running about 1755 RPMs. Sounds about right.