Two overdrives?
 

What would happen if you attached a secondary overdrive (say, a Gear Vendors unit) to an automatic transmission that already had an overdrive built into it?

You'll have double overdrive. The problem is the frictional losses go up with a set up like that and can possibly pull the engine out of its best operating range ( you're lugging the engine ), so you have to calculate where the rpm is going to fall when the second OD is kicked in. I learned in trucking that setting up a driveline that is geared to be most efficient in direct can save more fuel.

Why not go to gearvendors.com to see what they say about it?

+1

Isuzu had a model called the MPG+ in the 81/87 pups. the only change to the auto was the 3rd member and trans.
it was a 3.42 rear end with a direct drive 4sp.
the normal model was a 3.73 with a 14.5% O\D 5sp
fed emission MPG+ 2wd model
Compare Old and New MPG Estimates

fed emission normal 2wd model
Compare Old and New MPG Estimates


for what ever reason the CA spec MPG+ model has the 3.42 with the O\D 5sp.
there top gear is just to tall for a 60 hp engine, mpg is commonly less do to people just WOTing it in high gear and/or big tires.

I totally agree. The US spec vehicles are way undergeared most of the time so that people are forced to drive 55-60 mph for maximum economy. My old Honda Accord spins at about 2500-2600 rpms at 55, but at 70 you are at 3000-3200 rpms.

Bruce Malinson over at Pittsburgh Power ( builds hot rodded Class 8 truck diesels ) has brick like Peterbilt 389s with 800+hp Cat C15s that average 8-9 mpg loaded running single over 18 spds with 2.64 gears in the axles. Most modern aero style trucks can't do that.

Reducing friction and increasing thermal efficiency is all it is.

^ Yup. My Jeep is certainly capable of 25mpg or more at 60, but with the stock gearing turning 1750 at 60, there's not a chance of that happening, even with aeromods. Breaking 20 is a challenge :(

The 6 cylinder versions can see 22 - 24 mpg at 60 no problem, as they're geared similarly to mine, but with 2/3 of the engine displacement. With the bigger engine, taller gearing is needed. I should really be turning about 1400 - 1500 (max) at 60 for decent mileage. I probably get better mileage than the 6 when towing though, as I have enough torque to stay in OD on the flats and mild hills, while the 6 has to tow in 3rd pretty much all the time, meaning 2600 rpm @ 60 mph.

I currently read about 2050 RPM at 72 MPH. I figure that with a 0.78 overdrive, I'll go down to 1600 RPM.

Friction? I figure that friction from the propeller shaft back to the wheels should not change, and friction before the overdrive should actually go down. The overdrive itself, though, will add an element of friction. Whether that friction is significant enough to affect FE is another question entirely.

I'm not that concerned with lugging at 1600 at part-throttle while going down the highway, which is the only time when I would engage a second overdrive.

They have not responded.

I know what you mean, i have the V6 in my liberty and barely break 20MPG (combined at least).

definitely a different V6 than mine, i got 18-19 MPG while towing a fully packed 5'x8' U-Haul from Virginia to Illinois. accelerating with the trailer sucked, but once i was up to speed i cruised no problem, i could make it up decently graded hills and downshifted to 3rd about 3/4 of the way up.

Yeah, I was referring to the old 4.0 I6, not the 3.7 V6 you have. Plus, keep in mind, I'm talking about towing close to the max rating, not a 2500lb trailer.

And breaking 20mpg combined is good for a brick. I've only broken 20 on an all-highway tank once :(

You are adding another gearing system to the driveline, so the drag that it imposes on the driveline adds friction. And unless your engine produces the same torque level at 1600 as it does at the torque peak, you will not be operating at it's sweet spot, therefore a loss in mpg. Since you didn't say what vehicle it's going in, I guess it's your Dakota, so the aerodynamic drag is high especially at 72 mph. And you are using a gas engine with a throttle blade in the induction system so the pumping losses are high. It'd be better to do a ring and pinion change to get to where you want to go, a far sight cheaper and more expedient. But based on your numbers, the vehicle is geared pretty tall anyway (2050 rpm at 72 mph is slow for a gasser).

If you want to pull 1600 rpms at 72 mph and get great mileage, put a turbodiesel in that thing and shape the torque curve like a semi truck engine; or maybe alter the cam timing, ECM calibration and gearing to do the same thing with your existing engine.