Efficient driveline?

I've been wondering for a long while which transmission / drivetrain combo is best for the highway
This is rather dependent on engine output but say it is a Dauntless :v6: or 4.3
would a lighter mass stock 3 speed with overdrive and 5.38 or 4.88 be more efficient vs the popular yet heavy 4speed with 4.27 or 3.73s
final drive ratio being equal, ie cruising in top gear
Mind saying "low mass high driveline rpm" vs "high mass low driveline rpm"

now for a second example will i see a highway mpg improvement if i dump a t86/t90 5.38 axle for a sm465 4.27 or 4.88 axle
both behind a dauntless / 4.3 with overdrive
my mind is between "that's a heavy tranny" and "that's a good range of gears"

or what have you done and noticed a mpg improvement?

 

All of your 3 and 4 speeds have a 3rd or 4th gear ratio of 1 to 1 so it will all depend on your axle ratio, tire size will be a big factor in your gearing.

 

The costs of conversion will most likely outweigh and savings in gas mileage. Kind of like owning a diesel pickup. You have to be north of 150,000 miles before you actually start seeing payback on mileage due to the initial cost outlay for the diesel engine option.

So build it, drive it and enjoy it.

 

It's true that less rotating mass and lighter weight will be more efficient.

But please realize that the Jeep has very very poor aerodynamics, and your best way to improve efficiency is to lower your speed. Wind resistance goes like the square of velocity, so no matter how efficient your drivetrain is, you'll always be pushing that rock up the hill. Try it: (70*70)/(55*55) = 1.62, so 62% more drag from 55 to 70 mph. It's a formidable barrier.

 

x2

 

In theory at least, I think a higher (lower numerically) axle ratio would be more efficient. Even with an overdrive, the lower geared axle is still causing the transfer case and drivelines to spin at high RPM. In practice, I doubt that it would make much difference-I think Tim is right about wind resistance being a huge limiting factor.

 

I fully agree with Tim's statement.

You must first decide upon the slowest speed that you are comfortable with.

To get the most efficient use from any particular engine; that engine must be operated at its most efficient crankshaft velocity.
For the carbureted Dauntless this demands that the crankshaft spins at 2400 RPM.
The final drive ratio and the tire diameter are then calibrated to provide a specified speed.

I suggest Dauntless engine with standard 3.73 gears or 4.88 gears plus O.D. if you plan to cruise at or near 60 MPH.

 

I would suggest 4.10 or 4.27 gears in your differentials. The 4.10's would be closer to what you have if you compare overdrive ratio to 1:1 ratio if you are running 5.38 gears like I am.


And don't run over 60mph even with them. Your gas budget will thank you.

 

I dont know all the formulas and so fourth, but this is what I have. Its a v8 with a muncie M21 4spd with warn OD and 4.88 gears, and 35's.
I dont have a speedometer as of yet, but on the highway going with traffic being passed now and then, My rpm are right around 2400, 2500

 

You need to find a dynamometer chart for your specific engine.
Or else you need to verify the given crankshaft velocity at maximun torque output for your engine.
Maximum engine torque of xxx ft lbs @ xxxx RPM.
2400 RPM is typical for many but certainly not all engines.

 

Don't forget actual tire diameter plays a huge role here as well. A Jeep that works well with 3.73-1 differential gearing with say 30" tires may not work well with say 33" tires. Also add into the equation tread width and contact to the ground, weight, etc. can make a difference as well (rolling resistance and mass to overcome).

 

Another way to look at this is which drivetrain will give you more hp at the wheels for the choosen engien. For example a typical chevy truck with a 350 v8 and a th350 transmission will get better mpg over a th400 transmission. This is because the th400 is a more heavyduty transmission and sucks up more hp to move it, and people could see a mpg diffrence here. But when looking at manual trans for a CJ.. a t-90 vs a sm420, obviously the sm420 is bigger and more heavy duty. I dont know if it takes much more hp to rotate the larger manual over a stock CJ manual to see a diffrence in MPG. I think there are other factors that have greater effect on mpg over the choice of transmission that should be paid more attention to. Perhaps a bug sheild on the hood would improve the areodynamics enough to improve your mpg or 1" lift vs 4" lift.

 

I still think someone should try my hybrid idea. The belt drive on the t-case drum can go to an electric motor. When cruising you can turn off the engine and run on electric. when you need more power use the gas engine. or both at once. a couple extra alternators will charge the batteries. easy

 

It's MUCH more complicated than that....
Try it out and let us know how it works for you....

 

went from t86, 3.73's and 28's (got about 16ish mpg) to t14, 4.27's, Warn OD and 32's and get 18mpg.

good luck on the hybrid

 

as long as mpg does not drop noticeably...
I have a t-90 that is going, a Sm-465 at the parents, and a friend that needs a transmission to rebuild for class, so the options are, trans that might be ok once rebuilt or, for $3-500 more a transmission that will be fine.

 

Efficiency is typically opposed to durability.

The T90 will certainly be more efficient than the SM 465 due mainly to the extreme differences in the overall mass.
The SM 465 will certainly be more durable than the T90.
This is especially true if the engine force has been increased beyond that of standard.

In practice you will not likely notice any MPG gain or loss between these two transmission extremes.
If you still had a Hurricane engine the MPG difference would be more notable.

 

All good advice.

But really it's not going to gain you much.

Hatchback for the week's trek to work.bunny with a pancake

Classic Jeep for the weekend.:v6: