It's time to change my gears. The truck is an 87 6.9 C-6 trans with AST 085 turbo 4.10 rear. I would like to go with 3.08s if the truck will still pull a hill and tow a light boat. 3.08s would make my RPMs about 2150 at 65mph. Sounds good but I don't know, might be to low. With 3.55s, RPMs would be about 2440 at 65 mph. I think someone had 3.08s with a C-6, maybe it was David 85 before changing to an OD trans. Whats the way to go?

 

You won't have much trouble. My 83 had T-19 with 307's or 8's and it did fine. I wouldn't win a race up hill. It needed a gear between 3rd and 4th but at the bottom end it would pull stumps.

If your truck has a Dana 60 in the rear those gears won't fit. You will need a Dana 61 or go to a Sterling. There are gears for Dana 60's in the 330's or high 320's from what I have read on this forum.

 

I believe I've got a Ford 10.25 rear end. The only gears I've found it so far are 3.08 or 3.55.

 

I could really use some input!
Thanks

 

Sorry I didn't chime in sooner, I just finished my 6.9 rebuild.

To tow a light boat, the ONLY time you might miss the 4.10s is if you are on the boat ramp or accelerating hard with a heavy trailer behind you. Now that doesn't mean you won't be able to pull the boat up the ramp (we have a boat), all it means is you will have to give it a little more of your foot get get the job done.

For driving uphill at moderate speeds, like 50 MPH, you will be at least as well off as with the 4.10s. Heres why, with the 4.10s, gearing down at those speeds will make the engine scream and likely over rev, so you are stuck with 3rd gear. But with 3.08s, you can gear down to second even at 55 MPH rather comfortably, and the final drive ratio gives slightly more torque at the wheels with 3.08s and second gear than with 4.10s in 3rd. The difference in RPM is not much but you will turn higher RPMs with 3.08s in second gear than 4.10s in 3rd, hence the higher torque for 308s.

There is a moderate hill where I live that quickly builds up to 10% at the top, and is what we use to compare one vehicle against another (or powertrain configuration in this case). Heres what happened: With 4.10 gears, the top sustained speed on the steepest portion was 46 MPH (3rd gear), with 3.08s the transmission automatically kicked down to second, and I could coax close to 50 MPH out of the truck, not much but you can see the point I'm making.

What is different in yours and my situation than catfish101's situation is the torque converter because it will only accept power after a minimum RPM. That thing will take up much of the difference from the gear swap and keep nearly all of your drivability. For light driving you will also notice more consistent RPMs and lower noise. You might have to readjust your VRV, but thats not a big deal.

Now about what I mentioned about hard acceleration with a trailer. The difference is only felt BELOW 20 MPH, after that you will notice no difference in power or performance as you accelerate. For running empty, theres no change in 0-60 run time, off the line the feel is more of a big bungi cord yanking you forward instead of a solid smack you get with 4.10s, but you will keep up with traffic just fine. Again, this is largely thanks to the torque converter.

If you have a sterling 10.25" rear end, there are several sources for aftermarket gears. But if you have or ever plan to have 4wd, forget it, all you can get is 3.54 gears for the front end.

 

Thanks David. Still a little afraid of the 3.08s, I'ld like to find something in the 3.30 range but so far no luck. Do you have any idea where I might find other gear sets. Thanks again

 

Randys Ring and Pinion offers a 3.31 for the 10.25. http://www.ringpinion.com/PartsList....b-Differential

Jason

 

I was very surprised at how well the truck performed after the swap, but even going to 3.54 will make a difference. 3.31 was offered at the factory and might even be found at wrecking yards on rare occasions if you want to go that route. I prefer getting a new R&P though. There may also be a 3.23 ratio.