Gear Ratio Questions

[ChevMech]

I've been reading this site for awhile and finally decided to join.

 

I have a 94 GMC 2WD that is as slow as h*ll. I always thought it was geared extremely low because it runs 2100 RPM at 70 MPH, and has always been sluggish. I recently ran the RPO codes and found out it has a 3.42 rear end.

 

That being said, I have a 2003 Z71 with 3.73 gears in it and it is fairly quick and runs 1900 RPM or so at 70. Although I realize they are two generations of truck, the 94 Z71 I had ran the same as my newer one.

 

Could someone explain to me how this is possible? Someone has offered me a used rear end out of a 97 GMC for free and I was going to swap it out if the gears were higher just so my truck is quicker and better on fuel.

[tyhuck]

Are you the original owner? Maybe the 94 has had the gears changed. Plus the 2003 should have a torque converter lockup feature.

[ChevMech]

No I bought this truck used. I think I'm going to change it out. It can't make it any worse. From what I understand, the transmissions haven't changed in a number of years. They are both 4 speed automatics. The only difference is the 4 wheel drive.

[RyanbabZ71]

Higher gears will not yield better milage

 

 

Ryan

 

 

[Chris]

I thought they will in city but not highway. and what is the torque converter lock up

[kstruckcountry]

If it's free do it, if it doesn't help you aren't out anything.

 

The torque converter lockup is a clutch that locks the turbine of the tranny up so there is no slippage. Provides better mileage.

[ChevMech]

So my next questions is does the axle have a stamp on it to tell what it is? Since I'm not sure that this is the original rear end to begin with, I want to make sure it's as low as I think it is. Also, I don't want the next one to be the same thing and then I wasted my time on something that didn't have to be done to begin with.

[txab]

What tire size do you have on the '94?

 

Even if there were a stamp indicating gear ration in the diff, the gears may have been changed

 

Pull the cover and start counting

[tyhuck]

You could also jack up the rear end and unbolt the drive shaft. Then turn the pinion yolk and count how many times it spins in order to get the wheels to make a single complete rotation. If its a 4.10 gear ratio, the yolk should spin 4 times to get the tire to rotate one revolution. If it goes about 3 3/4 turns, then its a 3.73 ratio. 3 and a half times, then its a 3.42, and so on.