4 speeds

I put an SM465 behind a Chevy v8 with a Dana20 xfer case and I'm very happy with it. Granny is plenty low, but 60-65 is still comfy on the hi way. Mine originally was a 3 speed tranny and 232.

 

Exibit A ----> T15 = about 75 lbs while the T18 = about 160 lbs.

I believe there are just a few simple laws of physics at work here.
If the parts quality and the quantity were to remain constant then the DURABILITY increases proprtionate to the mass increase.
If the parts quality and the quantity were to remain constant then the EFFICIENCY decreases proportionate to the mass increase.

Primarily we all know that the complete drivetrain effects the overall EFFICIENCY and FUEL ECONOMY of a given vehicle.
That said the transmission is but only a portion of the complete drive train.

Bottom line is that the larger T18 must decrease EFFICIENCY and FUEL ECONOMY over that of a T15 yet the DURABILITY increases.

 

I agree on that, but at first, choose the right transmission for the use you gonna do with the jeep. If road only, never see off-road go with a t-15. For offroad use a t-18 is way better because of the first speed that give an impressive crawl ratio. Also in off-roading, this ins't a bad thing to have a heavier transmission it lower the center of gravity.

 

Second isn't synchronized on a T18? Thought it was but haven't driven one for a long time.

 

Only Low ( First / Granny & Reverse ) Are "Not" Synchronized on a Borg Warner T-18...........2nd, 3rd & 4th are..............Find a Borg Warner T-19 with or without the wide ratio and Low gear is Synchronized.........but good luck finding one.

 

I had one those T19's in an 83 F250 diesel I recently got rid of. I liked that tranny.

 

Sounds like these things are a lot of weight. Maybe if we took them out we could gain some mpg.

R)

 

Sigh, there's more to this than just weight and resistance. There is also the load placed on the drivetrain based on tire size, gear ratios in the differentials, etc. take a Jeep with 33" tires, 3.73-1 differential gears, and a T-15 transmission. If this Jeep is loaded down and used off road the load on the drivetrain can be much more. This frequently translates to having to rev and or load the engine much more just to get the vehicle moving. Step up to a heavier more inefficient transmission but lower gearing or better gearing match to engine and drivetrain requirements fuel economy can actually increase. I've seen it many times and have experienced it in my personal vehicles. The reverse can be true if the gearing and use are not matched. I re-iterate, it depends on the variables I posted above.


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Nick,
I did not imply to disagree with your earlier post.
In fact I support your concept concerning torque vs. load vs. gear ranging.
In my mind proper engine selection (torque output and velocity) is generally the prime consideration concerning jeep efficiency.
For optimum efficiency and fuel economy ALL drivetrain components plus the body design need to be calibrated in unison as a complete and synchronous system.

Concerning T18 vs T15; I am simply comparing transmissions only and not all of the related drivetrain.
Excepting for the T18's additional low gear the numerical ratios between the T15 and the wide ratio T18 are very similar.
Therefore when used strictly on road there is virtually no notable difference in gear ranging.
Excepting the use of the T18's low gear this implies that the same engines, the same final drive ratios and the same tire diameters can be used with either of those transmissions equally well.
Excepting use of the T18's low gear the T18 simply cannot be more efficient than the T15.
To be become efficient implies using a minimal effort meaning "less is better".
Simply stated it takes more energy to rotate the greater mass of the T18 compared to a T15.
Therefore the T18 will yeild decreased "on road" MPG if when all else remains equal.
That said the T18's increased rotational drag is only a minute portion of the vehicles total drag.
The total effect will be relatively small as in barely perceivable, unless the selected engine torque is very marginal to begin with.

 

The term "parasitic loss" ought to come into play here. Yes, it's mainly used to describe (one of) the downsides to auto trannies, but it also applies to any power loss due to the vehicle's drivetrain. This includes the range of most efficient to least efficient manual trannies in use in any of our jeeps. Same for the TC; it was (IMO) an real improvement when they came up with the dana 20, as it did not have the two 90 degree shifts in power delivery (is that a real term?) that the Dana 18 had. Instead the 20 delivered the power efficiently straight through (in 2wd) to the drive wheels.

It's telling that parasitic loss can be very pronounced; take the TH400 found in jeepsters, etc; it sucks up something like 40+ HP just to drive the transmission! Between that and excessive weight, it's no wonder our rigs get such poor mileage at times.

I've never owned a T18, but have owned several SM420s in older Chevy pickups. They routinely got much poorer mileage than the 3 speeds. That was partially because they tended to run 4.10 rear ends. But talking about mileage when running truck 4 speeds is kind of a dry subject, as that is not, never has been and never will be one of the 4 speeds strong suits. If you want mileage, put in FI, get rid of the parasitic loss and the excessive weight. And put in a T-5.

 

I brought this up because i want to build my dream jeep that i have been thinking about for some time. Now with gas so high i will probably stick to a 3sp to get all the gas milage i can. I will not use one of the newer 4 or 5 speeds.

 

For a ground up build the main decision effecting overall efficiency and fuel economy depends on specific vehicle model chosen.
The vehicle choice should be followed by selecting an optimum engine that fits the chosen vehicle.

To get top MPG out of a CJ and using only Jeep manufactured parts I'd probably build the following...

A short wheel base CJ with D-225 engine, T15 transmission, Tera low D20 @ 3.15, 3.73 final drive on 7.50 x 16" tires.

The problem is that efficiency and fuel economy nearly always reduces the CJ's offroad CAPABILITY.
Some exceptions do apply.
Choosing the smallest CJ possible is one example.
It simultaneously increases the overall efficiency and it also increases capablity for use in close quarters.

 

I'm thinking a bigger fuel tank should always help.

 

...and leave in garage...