Overdrive Options

BigBlockMopar said
I recently installed an A518 behind the 318 in my '73 Dart.
The 8-3/4" rear end has 2.93 gears in it, which I'm still using today.

Going from a 904 to the 518, I noticed the heavier drivetrain in the car, but I'm used to it now.

The motor is, of course, still way undergeared. On highway hills the car's speed drops back easily when in OD.
Only when doing 75-80 mph, the engine starts to get into it's powerband and the cars starts to move.

My mileage has dropped from an average 15mpg to 13mpg currently, but in the car's defence, the weather has become a lot colder aswell since I measured the 15mpg....

Is the mileage change that you are quoting from the highway/freeway or stop and go driving? An overdrive will not help stop and go, in fact, it might make it a little worse. But it should improve your freeway mileage even if you are geared too high. Example: I saw a nice improvement when I swapped from 2.71 gears with a 904 trans in a '70 Satellite wagon to 2.94 gears with a 700R4 overdrive, even though it was way under-geared. But that was all highway miles.