Seventh Gear!

[robchalmers]

desperately need an extra cog in my little diesel polo - only running 26mph/1000rpm in top.

so envious

[comptiger5000]

I'd love taller gearing for my Jeep. I turn about 1825 rpm at 60mph with stock tires and stock 3.73 gears. Going from the small-ish stock size of 225/70/16 to a 245/70/16 (optional size) will get me down to 1750 and give me a little more ground clearance to the diffs for offroad.

If it wasn't for the insane cost and amount of fab-work required, I'd love to swap in a T56 with either a 2.97 1st gear and the super-tall 0.5 6th, or the more common 2.66 1st / 0.5 6th, and regear the axles to 4.56 or 4.88, that would give me a stick, plenty of low end power, and still lower highway rpm (4.56 would be 1560 at 60mph, 4.88 would be 1670 at 60mph). If I kept the stock 3.73s and did that, my off the line power would be lacking a bit, and I'd be dropping out of 6th on most hills turning only 1270 at 60mph.

[Big Dave]

Couple of thoughts about comptiger's Jeep Grand Cherokee.

1. I don't think T-56 trannies can be fitted to a transfer case.

2. You don't want to slow the engine down by using larger diameter wheels/tires. That decreases MPG because it increases rotational moment of inertia of the wheels/tires. RMI goes up with the square of radius. They become bigger flywheels and you have to pour energy into them to accelerate or decelerate. Pickup truck guys find this out often to their sorrow. I've tried both ways - bigger and smaller and found this is something the factory boys get right.

3. A GV costs more than a T-56 but it indestructible. I looked into a T-56 but the vast torque of my diesel and the road load a pickup can develop (heavy load going uphill or pulling a traler for hours on end) would trash a T-56. Yes, I know T-56s are used in Corvettes and Vipers, but these arelight cars with powerful engines. They don't really put that much toque through the transmission. The GV was developed for a British Bedford deuce-and-a-half military truck.

4. Because it is so aerodynamically hopeless, your Jeep would be a great project just to show what you can do with gears.

5. Randy's Ring & Pinion sell Yukon R&P set for the Dana 44 down to 3.08:1.

What I'd recommend: Stay with stock diameter tires and go skinnier. "Pizza cutters" are generally fine snow tires. Then swap in 3.08s in the axles. Either that or stay with the 3.73s and put in a GV. That would yield you 2.98:1 gearing. That would cost you about $700 more than regearing two axles but you can cut it out when you like.

[comptiger5000]

I originally thought the same about mounting the T56 to a t-case. However, it's apparently been done before with adapter plates and/or a different tailcone.

A stock T56 is rated for 400 - 450 lb. ft. of torque, my engine puts out 345. So that even leaves a little room for growth, especially if I used a beefier aftermarket T56 rated for more torque. For around town mileage, if I were going to do anything, I'd like to ditch the auto tranny. The t56 would give me that, and a good ratio spread. I could also do an NV3550 or something + a GV. Also, I know that I can get gears for both my D44a rear and D30 front in 3.08, 3.55, 3.73, 3.92, 4.10, 4.56, 4.88, and maybe 1 or 2 other steps.

As far as the tire size, the purpose of the slightly larger tires (about 3.8% taller) wasn't to get the RPM down. That's just a bonus effect. Plus, they're only a little heavier. (31lb for current rear tires, unsure of fronts, 33 or 35 lb for planned new tires, depending on which I get, either Toyo Open Country A/T or Firestone Destination A/T). The weight hurts mostly around town anyway, and the mileage is so hopelessly crappy around town, I don't think it would make more than 0.1 mpg difference.

The only aero mods I have planned for now are some not-too-heavy skid plates to clean up the underbody air flow a bit, and offer some protection down there. Otherwise, I plan to do it all with gearing and engine mods (better injectors are said to help, and about $140).

Bone stock, I was able to average 19.2 mpg last weekend running on the highway (no trucks) at 65mph average, including 2 miles of stop and go traffic, with the A/C on the whole time, and with some rain.

[nubbzcummins]

WOW! My truck turns 1450rpm at 60mph and 1700rpm at 70mph. This is with 325/55R22 tires and 3.73 gearing with my 4 speed auto (48RE) tranny. Mine runs around 1-2psi at 60mph on flat/level road. I've been getting 23mpg+ tanks lately with no aero or really much eco driving at all. I can't wait to see the improvements you will have with the GV in that ford. BTW mine will do 35mph at 850rpm locked in o/d.....up grades it will smoke a little though!

[comptiger5000]

I've got a 46RE (same ratios as 48RE) and 3.73, so the only gearing difference between yours and mine is the tire size. I know mine will climb a hill and have enough power to just start to gain speed before I hit the downshift point in OD locked at 1100 (36mph, any slower and it unlocks the TC). It's good enough to have surprised a few people that it did that and isn't a diesel.

I could probably lug it down to 800 locked up on the flats at 30 - 40 mph. I wouldn't want to go below 1000 - 1200 on the hills though.

[Big Dave]

Sorry about taking so long for results.

Had some alignment issues. Truck trying to shake itself to bits. This divorced gearbox setup has a lot of scope for misalignment.

Should get tests going shortly.

[bhazard]

Boooo I wanna see some numbahs!

[Big Dave]

Better a lackanumbahs than listening to me bellyache about broken-down machinery.

It seems to be fixed now. I can start testing after the Fourth.

[slowmover]

Came across a guy today with Cummins 4bt conversion into his 2001 1500: 27 city & 33 highway. Teething problems but nice work to view:

Pics of truck - Dodge Cummins Diesel Forum