Engine/Transmission (1994 - 1998) Gear ration question

[Rick Harper]

I recently put a G56 in my 97 which had a auto in it. In my research I must have missed this...



With the auto I was turning about 2050 rpm at roughly 70mph. The G56 I put in was from a 2006. Now I have to push to 2250 rpm to get 70, and that is after a tire swap to an overall larger diameter. This is annoying, loud, and mpg has significantly suffered.



What are my options? Based on my new research, I would have to go to a 3. 27 diff ratio to get the rpm back to 2000 at 70.



Is there such a gear ring set for this truck? Are there other solutions such as an overdrive unit?

 

[HBarlow]

Rick,

The only practical solution to your dilemma is to swap your G-56 for the later version that has a higher (lower numerical) overdrive ratio. Or change the gearing in the G-56 you have. Later G-56 transmissions used a different set of gears to improve the overdrive cruise rpm. The various ratios have been published several times here in the TDR website but I don't remember how to guide you to a link.

The old 47/48RE automatics were slushboxes with lots of limitations but they did offer a great overdrive ratio. You basically downshifted one gear with your G-56.

To my knowledge there are no 3. 25 gear sets available for the Dana 70/80 rear end you have and the aftermarket add-on overdrives are not products I would buy to hang behind a Cummins engine.

 

[KBurgoyne]

manual transmissions always run about 2-300 rpm higher than autos. the lowest gear ratio for the dana 80 is a 3. 31. if you want to check your math, west coast differential has an rpm calqulator on their site thawhere you enter your gear ratio transmission and tire height.