3.08 Ratios in a K/10 Sub

[LH Lead-Foot]

I chose to go with a 3.08 gear in the "GM" 12 bolt truck diff on my Eaton Posi. The 8.85" carrier has to be a type 3 for correct set up. There is a split in ratio's from type 3 to type 4 that many have with 4.11 gears. I was lucky as I have a diff spreader.
Here's my math with a 700r4 / 4L60 / 4L60e with an 0.70 over drive. With P235/75R16, that tire has a 29.88 diameter. At 60 MPH, in over drive, 3.08 final + tires=1454-1504 RPM with lockup. This should get the MPG up there in the mid teens or higher.

[LockDoc]

Originally Posted by LH Lead-Foot

I chose to go with a 3.08 gear in the "GM" 12 bolt truck diff on my Eaton Posi. The 8.85" carrier has to be a type 3 for correct set up. There is a split in ratio's from type 3 to type 4 that many have with 4.11 gears. I was lucky as I have a diff spreader.
Here's my math with a 700r4 / 4L60 / 4L60e with an 0.70 over drive. With P235/75R16, that tire has a 29.88 diameter. At 60 MPH, in over drive, 3.08 final + tires=1454-1504 RPM with lockup. This should get the MPG up there in the mid teens or higher.

If you are going to try and use the 3.07 with a 4l60e you will have to run a trans fluid cooler. I tried it in my '69 Suburban and the trans overheated and blew out the front seal. I found that the 3.07 is too high for a 4l60e, it puts too much stress on the trans when in OD and locked up. I ended up putting a 3.73 rear in it and still get 20mpg.

LockDoc