Gearbox Upgrade

[Tony302]

Hi Everyone
I have a 1979 351 G30 Coachmen Surrey, in the UK
I get about 14 mpg out of my 3 speed automatic gearbox, does anyone have experience of installing another one, say 6 speed?
I am aware of overdrive upgrades, but would like a lot more
Thank you
Tony

 

[Kevin D Pem]

When towing or hauling the recommendation is to lock out overdrive. A two speed differential would serve better. The advantage of multiple speed trannies is the perfect gear selection/RPM range. This allows for a steeper/lower gear ratio for more torque climbing hills.

On my RAM 8 speed ZF transmission, the overdrive is not used while towing, effectively making it 6 speed.

I average 20 mpg when not towing, but 13 when I do. To experiment I have put in tow haul when not towing and got similar results gas wise.

Conclusion:
New computer controlled engines are the best way to increase gas mileage. Any upgrades only serve to get the engine at optimum preforming RPM range (better hill climbing ect.).

 

[Tony302]

Hi Kevin
Thank you for the reply
Introducing computer controlled technology into my old Chevy sounds a little over the top

 

[Kevin D Pem]

Hi Kevin
Thank you for the reply
Introducing computer controlled technology into my old Chevy sounds a little over the top

I agree but I wouldn't be complaining about 13mpg, that is my mileage pulling a trailer of any size with a frontal area, using my modern 16' RAM. Advantage comes when not towing 20 MPG.

A two speed rear-end could maybe make the best of things, but shifting would be by shifting the automatic into neutral every time you shifted the rear.

 

[Rich W.]

Hi Everyone
I have a 1979 351 G30 Coachmen Surrey, in the UK
I get about 14 mpg out of my 3 speed automatic gearbox, does anyone have experience of installing another one, say 6 speed?
I am aware of overdrive upgrades, but would like a lot more
Thank you
Tony

A six speed typically has at least one over-driven gear with 5th usually being 1:1, with some having two overdriven gears with 4th being 1:1. Since you already have 1:1 in third, and don’t want overdrive, swapping to a 6 speed (if its even possible) would really only net you one extra gear, closing up your ratios a bit which might improve performance under some conditions, but since you wouldn’t be using over-drive much, probably wouldn’t improve your MPG, which as Kevin said, isnt all that bad anyway. And if you did want to improve mpg a bit, an OD unit (such as a Gear Vender OD at around $3200) would do it for you and would be a fairly easy job.

 

[Tony302]

Thank you Kevin, Rich
The more I investigate this, the more I am learning
Currently I have a TH350 gearbox, the later generation of Chevy has the 700R4 gearbox, which will bolt straight on with a reduced prop shaft
This will give a top ratio of 1:07 which is equal to 6 speed boxes that I have seen
Now I need to find one and get it shipped to the UK

 

[GypsyR]

It would cost more but if it were me I'd be looking for a "4L80E". Not only will you gain a fourth overdrive gear but they are WAY tougher than a 700R. 700R's are pretty iffy in half ton trucks if you ask me. 4L80's can be found in 7.4 liter 20, 000 pound class A's over here. And they work fine I don't believe the factory put any 700R's or their electronic successor, the 4L60E in any truck or van that is rated as being a one ton like a G30. They evidently didn't think they were robust enough either.

To use a 4l80E, among the other changes, you would have to buy the standalone computer and harness that makes it work without a factory computer. Those run right at $500. Not a cheap undertaking all told and I doubt you save enough in fuel to pay for all that. Even knowing your petrol is a LOT higher in price than ours.

Part of my shopping criteria when I was looking for my older coach (1993 model) was that I wanted it to have overdrive. It does. Spends a disappointingly small amount of time actually in it, but it's there.

 

[Tony302]

It would cost more but if it were me I'd be looking for a "4L80E". Not only will you gain a fourth overdrive gear but they are WAY tougher than a 700R. 700R's are pretty iffy in half ton trucks if you ask me. 4L80's can be found in 7.4 liter 20, 000 pound class A's over here. And they work fine I don't believe the factory put any 700R's or their electronic successor, the 4L60E in any truck or van that is rated as being a one ton like a G30. They evidently didn't think they were robust enough either.

To use a 4l80E, among the other changes, you would have to buy the standalone computer and harness that makes it work without a factory computer. Those run right at $500. Not a cheap undertaking all told and I doubt you save enough in fuel to pay for all that. Even knowing your petrol is a LOT higher in price than ours.

Part of my shopping criteria when I was looking for my older coach (1993 model) was that I wanted it to have overdrive. It does. Spends a disappointingly small amount of time actually in it, but it's there.

Thank you for this
The 700R is definitley off the shopping list now
Just Googling the 4L80E over here, it was also fitted to Rolls Royce
There was one sold in the past for £500, but doesn;t say how long ago that was, so will keep looking
From your knowledge, will the 4L80E fit my G30 without any adapted plates, electrical configuration noted, I imagine the prop would have to be shortnend an balanced, but that is nothing?
I notice it also fits Fords, so I might consider it for my RHD 1978 Mustang 302 in the future

 

[Kevin D Pem]

Thank you Kevin, Rich
The more I investigate this, the more I am learning
Currently I have a TH350 gearbox, the later generation of Chevy has the 700R4 gearbox, which will bolt straight on with a reduced prop shaft
This will give a top ratio of 1:07 which is equal to 6 speed boxes that I have seen
Now I need to find one and get it shipped to the UK

Whatever you get you will need to make sure it is not computer controlled. That of course is unless you have the ability to program the controller.

 

[GypsyR]

There are computer "kits" for the more popular transmission retrofit swaps. Usually from $500-750 for a standalone module, wiring harness, and various bits to make it work. Once again, the 4L80E kit for this application runs about $500.

Though Rolls and even a couple of mid-1990's Jaguars used 4L80E's, this doesn't mean one will bolt to the back of your engine. They don't have swappable bellhousings. The bellhousing part that bolts to the engine is the actual case. I know they made a few different cases for them. Now whether they made some specially for the RR and Jaguar applications or they were a standard Chevy configuration I frankly do not know. Not so many of those cars over here. Ideally you'd want one from behind a 5.7 Chevy/GMC truck or van.

I've not seen one in a Ford. You could do that but it involves cutting the bellhousing part off the case and machining bolts holes to attach a custom bellhousing. Such bellhousings run $500 each. No idea what the case machining would cost.

You have a Mustang II? Best fit for adding overdrive to one of those is to swap to a 5 speed. Reason being that the automatics are a bit large and the car is small. Manual transmissions tend to be slim and trim. The go-to automatic overdrives for those are the AOD or the 4R70W. I haven't done one myself on a '74-78 car (my Mustangs are older and newer) but I understand you have to modify the transmission tunnel a bit (big hammer) and trim off some non-essential protrusions on the transmission case. And it will still be a very tight fit.

I like AOD's for being more simple and not requiring a controller kit but for the trouble I suspect you might have to go to anyway I recommend spending a bit more for a 4R70W setup. Depending on what you can actually find, of course. No way you'd ever recover the money it would cost with the gas savings in a car. But when you are on the highway and it drops down into overdrive at about 50 mph it sure is sweet. I'm addicted to overdrive and want it in everything nowadays. Manual or automatic. I bought a VW Caddy years ago (driving it today as it happens) and one of the first things I did was source a five speed from a Golf to swap into it.