Granny 4 speed is way too far between gears to actually work at optimum. If you want a real poorman's OD, slide a gutted 3 speen turned around backward behind yout other transmission. I would loose the 1st/reverse cluster and use 3rd for 1:1 and second for OD.
One of my buddies has that setup in his mid 70's Chevy 4x4 that he has a 454 in. I'd have to ask what rear gears he runs though. His truck is a dualie conversion that hauls a camper and he claims a pretty decent mileage improvement with the two speed OD working. It is big and heavy though, I picked it up and moved it one day when I was doing something in his shop and wished I hadn't.
The one advantage of a setup using available a wide ratio 4-spd car trans, is that with the right final gear, you will be 1:1 thru the gear box, so minimal energy loss there. Slightly better mileage, if the other factors the posts bring up are well chosen.
I have an OT diesel pick-up with a 5 speed (manual) Alison box...granny gear in first....overdrive in 5th...often wondered how it might work in a rod. The diesel operates at a much lower RPM (redline is 4000).
I was having a gears talk just the other day so this is fresh. I had an O/T Mustang that I swapped the pumpkin with a set of 2.86 gears. A dog off the line but I wasn't racing it. Straight 6 and a 3 speed. I got about 25 mpg. Diesels are another game, but same principle. 5.9 Cummins as an example is very happy between 1800 and 2100. Consumption increases dramatically on either side. Back on topic, A poor mans overdrive is probably not the right term but tall geared rears to get the rpm is what I am getting from this thread. I say yes and in normal circumstances, you will get improved mileage and better highway streetability. I know from the Mustang and a 3 speed, but same concept.
The Jalopy Journal
