Overdrive transmissions have been around before WW2 as a means of improving gas mileage.
New 1955 trucks had the option of electronic-overdrive.
1956 U.S. Interstate Highway System was funded for higher speed travel.
From 1955-65 electronic overdrive transmission did not have synchros in 1st and 2nd gears.
1966-69(?) fully synchronized electronic-overdrive.
~1983 overdrive was incorporated into the gear set of a T-5 transmission for Chevrolet S10.

Short and sweet, the electronic overdrives are nice but expensive. Usually you find the transmission without the solenoid, kick down bracket, kick down relay, governor, pull handle for manual operation etc. Typically the electronic overdrive is used on restorations. There is a ton of information freely available (let us know) but most choose to go with the S-10 T-5 for the simplicity and readily available repair parts.

Last edited by buoymaker; 08/09/2022 3:08 PM.
"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."