Has anyone swapped an overdrive unit such as the 700R4 or 4L80 in place of the stock 400. Any improvement in MPG would be a plus with gas prices on the rise again in SoCal.
Thanks,
Garry

I woudln't do it... simply for the fact that while both those trans are good... you can't beat a Turbo400 for overall strength and relaibility....

start pushing high torque and often WOT runs... you'll kill the 700R4 in no time...

stick with the TH400.... these cars have a very high final drive ratio because of this....

If you really want to get better millage, look into the cam that... IIRC MTS? sells.... they claim you can get up to 22mpg with it...

otherwise don't drive the '73 DeVille... that and gas millage kinda contridict themself :)

I would disagree that overdrives can't handle massive amounts of torque. If properly setup, trannies like 4L80E, 700R4 and even 2004R can live happily behind big blocks. Check out http://www.bowtieoverdrives.com ,read up and see for yourself.

Another option is make the TH400 O/D. mmm 6 speed...

http://gearvendors.com/

If you Really want an OD tranny the only one that will live and be fairly cheap is the 4L80... if you need a computer to control it look into the diesels that had them, they had a standalone computer you can find in a junkyard for fairly cheap.

Gear splitting the 400 with the gear vendors unit is an expensive but really cool option.

What the hell can I do to get better mileage in my Silverado. She pulls like a Russian race horse, but drinks like a whale. It's got the T350 (?) the 3 speed anyhow.

Thanks for the response's. Son's on a limited budget with school and part time work. It is a very cool car to drive though. We will probably change out the cam. been looking at that option for a while. Especially since I hope to have my 67 Chevelle completed this weekend so I'll have some time to play with it.

I'm very curious about this question as well. What I know so far: You wil have to lengthen/shorten the driveshaft, buy a bellhousing adapter, and to control the electronics,you will need to buy something like this:
http://www.powertraincontrolsolutions.com/content-4.html
What I don't know: Will you have to fab as new crossmember, and if so; how-so? According to references here:
http://www.transmissioncenter.net/TransmissiomMeasurements.htm
The transmission measurements are very different. I hope that someone who's put a 4L80E where a TH400 used to be will chime in. I hope that my contribution provided some insight, and I will be watching this post to see if any ground-breaking info comes up. PRG04