I have the 4..8/4l60e in my truck, was told I had 3.08 gears in my 12bolt truck 3series rear end.

I put a tack in the truck resently and when Im driving down the highway at 70, it shows 2250rpm, which if you use one of those rear gear calculators show I have a 3.73, am I reading this right or is the tune wrong because I have 27.1" tires ( 255/60r15) and the (?) 3.08 gears with .70 final drive ration on the 4speed auto.

Can yall help me out some?

Thanks,

 

Sounds like 3.73's...
Look on the rear for a tag, or jack up spin the tire 1 revolution,and count the the revolutions of the driveshaft (with motor off and tranny in neutral of course)

 

That was my next thing to do, just was curious if the PCM had anything to do with the tach input because of the tire size/rear gears/etc. I know it needs that for the shifting and such but what about the RPM's?

 

If the converter is locking up as it should, then 2250 rpm at 70 MPH w/ 255/60R15 tires, then that figures to be a 3.73:1 axle.
But if the converter is not locking up, then it could well be a 3.08:1 gear.
Best to pop the cover and verify the ratio, and while you're there, give it some fresh gear lube.
If you confirm 3.08:1 gears, then you know there's a serious problem preventing the converter from locking up as it should.

 

RPM should come straight from the ignition pulses or cam/crank sensor. Rear gears and tires don't change when the plugs need to be fired.

However your speedo could be off. The PCM can adjust for that.

Easiest solution is jack up one rear wheel and putting a piece of tape on the driveshaft and tire count the number of times the driveshaft spins when you turn the rear tire one full turn. Divide that number by 2 to account for the diff. Should be easy to tell between a 3.08 and 3.73.

 

What kind of tach is it? Old factory tachs are known to not be very accurate. Have you put a scanner on it to see what the RPM is in the PCM?

 

The rear end was rebuilt about 2.5 years ago before I did the LS swap. I drove the truck to work this morning and it drives fine but I am not for sure if the lock up is actually happening. I have an aftermarket tach and it is reading pretty close, I had a mechanic friend check it with his device but I didnt see it, he just said it was close.