[Formulajones]

If you're serious about an overdrive, Shiftworks sells all the conversions for different factory automatic shifters to work with the new transmissions.

https://shiftworks.com

With the addition of overdrives you can expect about a 30% or more increase in gas mileage.

My wife's camaro went from 17 mpg highway with 3.55's and a muncie to 21 mpg highway with a TKO600 5 speed.

My Chevelle went from a little over 12 mpg highway with 4.10's and a 400 turbo to 16.88 mpg with a 4L80E and this car makes a little over 600hp. It's now a nice cruiser.

It'll completely transform a car. Like Formulabruce has shown it cuts the rpm cruising down to an area you just can't get with a 1:1 ratio at those speeds.
Not just better mileage and less wear on the engine, but less noise in the cabin too.

The biggest drawback is cost.