What rearend to use ?

[D-dog357]

I have a 3.07 sitting around and was wondering if I should use it or not. With my 3.54 now it seems like going down the freeway at 65 I could easily go up another gear as long as I don't haul a load. I have a tight TQ and realize that I would have to forgo OD when hauling but most of the time I run empty and would like the best mileage I can get on road trips.

Is anybody running a 3.07 and a slop box w/ OD ?

What would it do to my TQ when I pull a load locally, no hills just flat - say 12k GCVW.
I'm concerned it might eat-up it up in short order from the extra stress of getting it rolling.

As always thanks for any input.

Also - can someone please tell me the gear ratios of the 518 trany ? I tried searching but way to many results to go thru, I gave up.

[RCCUMMINS89]

From what I'm reading you'd love the 3.08 gearing.

However, 12k with 3.08 will feel like 15-20k I bet with 3.54s.

[Mark Nixon]

An A518 or 47RH O/D ratio is .69, so 69% overdriven.
That's not AS much as say a Getrag, NV4500, NV5600 or either of the G-56s, so it may work out nicely, since it spins the engine slightly higher RPMs in O/D.
If you're infatuated with the automatics, do yourself a favor and swap to the 47RH with a dual disc torque converter, it'll really tighten things up and the lock-up is such a nnniiiiiiccccee feature, especially if you tow a lot and put it on a switch.

As far as the 3.07s go, remember this one fact:
The taller your effective rear ratio, the heavier duty clutch (clutches in an auto) you'll need to take the transmitted torque from a stand still start.
Taller tires/gears= more load at the engine.

One way to overcome this is to have a lower first gear ratio.

Mark.

[u2slow]

Sounds like you use your truck about the same as me.... mostly empty, but the occasional heavy load. I have 3.55's with getrag, and yearn for lower highway revs. The 518 has a very tall OD, taller than any of the manuals. Your 3.07 becomes 2.11 final ratio with the .69 OD.

I would simply not use OD when hauling, or only as grade conditions permit.

[cmann250]

At one time, I did have an overdrive automatic (47RH in my case) and 3.07s. I never towed with that set up, but I would absolutely believe that it would be hard on the truck.

I tow frequently with my stock, non-O/D 727 and 3.07s. It's my opinion that it's not bad at all. I suspect my tractor and gooseneck are in the 10K-12K range, so that's right in the ballpark you'd be in. With 3.07s, towing in 3rd is quite livable.

[D-dog357]

I understand that towing any kind of load and on hills would be in 3rd only.

Sounds like it's a new project for the fall I guess as it's going to be 100*+ for the rest of summer. Might as well go through it in the mean time. Any advice on what to look for when I pop the cover and what if any mods to do to it ?