More info:
"The GEAR VENDORS in a combination with the Powerglide is an affordable, reliable, and very efficient package. This is a superior alternative for most cars looking to go to a 3-speed. The GEAR VENDORS .78 to 1 ratio allows you to go to a numerically higher rear gear to improve your 60ft. times and get it all back with the overdrive. The reciprocal of our ratio is 1.28 so a car that currently runs a 4.56 rear could go to a 5.84 without loss of top end. You will have these improvements with reduced cost, greater reliability, and lower parasitic losses when compared to 3-speeds. Using a lower rear end ratio also lets you keep a close ratio spread on your glide for more power at the 1-2 shift. Most cars with 1200hp or less will also find benefit in using 1st-over which will net a really nice 4spd spread of 1.76, 1.33, 1.00, 0.78.
The overdrive is switched on with 12-volts and so is easily adapted to steering wheel buttons or automated through our Auto-Shift sequencer. The product is ideally suited in stock form for cars with 1200hp or less and in this case is virtually indestructible, literally running for seasons of use without maintenance other than fluid. Cars with more than 1200+ horsepower can be accommodated but you should call the factory so hydraulic issues dealing with clutch ramp timing can be properly adjusted for your application weight and hp."
-directly from the GV site