Actual mileage advantage THR700 overdrive automatic?

[Wendell Hanks]

My '79 has a 3:07 differential and a GM Th400 short shaft transmission.

Have any of you replaced your TH (GM Turbomatic) 3-speed transmissions with a TH700R 4-speed to obtain increased gas mileage?

If so, was there any substantial difference in city driving compared to the previous trans?

My '79, with the replacement GM350 crate motor with 35,000 miles averages 15 mpg in the traffic-clogged Los Angeles basin.

If a replacement, I know Myers Studebaker has a "kit" that enables the replacement -- dealing with the new crossmember and the electronic substitutions.

Any insight and specific mileages are appreciated if you have made such a replacement or have an Avanti year that came with the TH700R 4-speed.

Thanks,

Wendell