"All the overdrive is gonna do is LOWER the Engine R P M"
Yes an OD transmission will lower engine RPM at cruising speeds, but it will not make as much impact at lower speeds as changing the rear gears would.
My point... 3.38 gears can't work as hard as 3.90 gears for pulling loads. 3.90 gears with an OD are more economical than 3.38 gears, as the final gear ratio with a typical 27% OD is equal to about 2.85:1.
In mathematical terms, a 3.38 rears will reduce rpm by about 13.3% in every gear. This reduction of rpm in the lower gears is not desireable. A 3.90 rear with an OD transmission will not suffer much in the lower gears, but the 27% reduction in OD is very desireable.
Here's the disclaimer: If we are talking about the sm420 4 speed transmission being replaced by any OD transmission...there will be a loss of that granny gear. The first gear of the sm420 transmission is around 7:1. The only OD transmission I know that comes close to that low gear is the NV4500 with a typical 5.61:1 first gear (early gm versions boast a 6.34:1 first gear).
