For the guy with the E250, the cost of even a do it yourself overdrive may not even justify the fuel savings. Back in the 70s, a friend had a relatively new IH 3/4 ton 4x4 pickup, 4.10 gears. He was looking for better mileage, so he picked up a Hone (I think) overdrive. Even running empty, the best improvement he saw was about 1 mpg.

You will probably get similar results, as you are also pushing a big box through the wind.

Using a 3 speed backwards is an option that could work, but here are a few things to think about. Lubrication will not be an issue as long as you keep the top up and the bottom down. The reversal of rotation might be an issue in that it can reduce bearing life. Just about any 3 speed you will find that has the torque capacity you need will have helical gears. Reversing rotation of helical gears changes the direction of thrust on the bearings, and as you will likely be using second gear as your overdrive, this changes the direction of thrust loading on both main and countershaft bearings. Another thing is that these transmissions were designed to spend most of their life in third (direct) gear, where the countershaft is just idling. Using second as overdrive will keep the countershaft loaded for extended periods, something it is not designed for, especially with the reversed thrust load.

 

Yes I believe you are right-adding any type of overdrive at most might give me a 2-3 mpg. savings which does not justify the cost. And after thinking it over I have the same cncerns about using a 3 speed backwards.
But I just wish there was someway to save at the pump-if not I may not be able to afford to drive this big orange Block.
It's time our government steps in and starts refineing and keeping our oil here rather than making the Saudi's richer and richer !!

 

I'm also looking at overdrive alternatives. I just had a Gear Vendors self-destruct. The sun gear exploded and apart from a couple of pieces of teeth, all I found were filings. This was a divorced unit mounted in place of the centre hanger bearing. Even if I decided to do without overdrive, I'd still have to fabricate a hanger bearing and shafts. The present shafts are now too short and would no longer fit a stock hanger bearing. I thought I had it made when I found a Mitchell, but it's too far gone to repair without replacing parts (which aren't available).

I had thought about a transmission running backwards some time ago as part of another project, but rejected that idea for the reasons already discussed in this thread: unsupported input shaft and possible lubrication issues.

Reading this thread, though, another possibility occurred to me. What about using a 4-speed overdrive transmission running the right way? It wouldn't be much bigger or heavier than the Mitchell, never mind a Brownie. It would solve any lubrication issues, but it would still have the unsupported input shaft (if that really is a problem). I could just use overdrive and direct, with the option of also using second if I decided later that a creeper range would be useful. It would also give me a neutral, in case I ever have to tow the motor home.

It used to be a common practice as far back as the model T days to use 2 automotive transmissions in tandem to get super-low gearing in off-road trucks and farm machinery, so there should be lots of information on how people did it. There was also a company selling marine conversion kits in the '50's that kept costs down by converting automotive transmissions to marine reduction gears. As I recall, they used 1st and reverse, and they must have added a thrust bearing. The only difference is that I'm proposing to us the high gears instead of the low ones.

Does anybody have any opinions about which 4-over transmission would work best? It obviously would have to be strong enough. It would also be nice to use something fairly common (i.e. easy to find and cheap), and not too heavy. There are also 5-over transmisions, but they're likely to be either too flimsy (T5) or big, heavy and expensive.

 

A NP 435 would be strong enough. They were used in 1/2 tons up to 2 ton trucks. They have a cast iron case and an aluminum top. Weight is around 140 pounds. Some Dodge versions had a 4 something to 1 first gear.

 

I thought the NP435 had direct 4th. What application would I look for to find an overdrive version?

 

They do have a direct 4th. Some 435s in Dodges had a higher first gear ratio. 4 something to 1 as opposed to 6 something to 1.

 

There were a few 4 speeds with overdrive 4th back in the 80s. The one I remember was used in the F150s. They never did seem very common around my area. A friend had one in his mid 80 F150, and the transmission was rather light duty. The one thing that I remember was that the owners manual recommended that when towing use 3rd, not 4th.

 

Sorry, I glossed over the part about you wanting a 4th over tranny. In the early 70s IHC offered a 5 speed in their light duty trucks. Kind of rare and sought after by the IHC crowd, but worth peeking in the cabs of any derelict IHCs laying around.

 

So it looks like the choices are pretty limited in 4-speeds. It sounds like the Ford top-loader is too flimsy. The only other one I've found is the Chrysler A833 ( aka NP440) and I'm not sure of its strength either. Does anybody know of others? It looks like all the good overdrive transmissions are 5-speeds and too heavy and expensive (NP 4500, Getrag, ZF) There are possibly some older medium-truck boxes that I haven't come across. Has anybody else found any?

 

Don't know about Ford top-loaders being too flimsy. The nasty R-code 427s of the early to mid 60s were often paired with the Ford top loader. Can't speak about the newer ones, though.

 

I had a friend who drove a 60s falcon wagon, it had three on the tree and a overdrive that resembled a PTO unit behind the transmission, he clammed it was a stock Ford unit

 

You'll be miles ahead and, by the time it's all said and done, probably cheaper to just do it the right way. Change the gears in the axle or find an axle that better suits what you want it to do. Piddling around the neighborhoos in cobbled up crap is one thing, safe, reliable transportation is another.

 

I believe he was referring to the SROD/T170FS and TOD/T170FT that were in '80 - '85 F-150s. They were not up to much abuse, but had an OD 4th gear.

 

Truck sales engineering 101. Aerodynamics. Why trucks are streamlined and use air deflectors. Frontal area resistance and eddy current drag (behind cab) = drag coefficients.
Since your truck has the aerodynamics of, say, a cinder block, lets assume a frontal area of 49 square feet (7' x 7').
According to the Wallace Racing calculator:

Using your vehicle Speed of 80 MPH takes 171.27 HP to overcome air drag.
Rolling resistance is 8.32
For a total of 179.59 HP to run 80 MPH.

There you have it! So for whatever drive train you decide on I'd make sure you get the rev's to develop that kind of power. And this does not take into account the parasitic losses either.

Calculate HP For Speed

 

I would be concerned about the reverse thrust on turning a transmission in reverse. The thrust bearing are designed for input on the one end not the reverse. Also as noted the drive shaft and rearend gear rpm's would be a major concern.
Bruce