My 1990 GMC 7.4 Suburban TH400 transmission and torque converter are going out, big time, w/ only 110,000 miles. Instead of replacing the TH400 3 speed I was considering a GM 4 speed that was offered on later Suburbans. I'm thinking I might be able to improve on my 11.5 mpg on the highway.

1) Would the newer style transmission bolt in or require significant modification?

2) Any idea on cost for a remanufactured transmission, say from the GMC dealer?

3) Any other ideas/swaps that might make sense with the goal of getting even slightly better gas mileage?

Thanks in advance!

Eddy

The 4L80E is the obvious choice, since it was standard-issue in the same vehicle for '91.

Do a search for 4L80E... the topic is covered monthly.

A rebuilt TH400 is cheap. IMO, you will be driving for many years before you'd recover the 4L80E upgrade cost in fuel savings.

You can also change the axle ratio for better highway economy. In my experience, it won't affect in-town mileage, but it will lower your towing capacity. You can get a complete used 3.21/3.42 ratio axle for $100-$200... it doesn't have to be a costly visit to a differential shop.

You can rebuild the 400 and buy a gear vendor unit probably cheaper than buying a 4l80e, having it built and buying the harness for it.

4L80E better milage Should be able to get a core locally for around 400 bucks good rebuild 500 bucks! But the kicker is you need a controller for the transmission more money!

Thanks for the responses, guys. I think I'll just rebuild the 400 and drive slower.:)

stick with 400 get a big ass cooler for it and use good tranny fuild


then just burn rubber all day

You can also put some bigger meats on the rear.... I run 33's on my C20 Burb for the slight edge on economy. ;)