Overdrive Options

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.

I would like to swap over to 3.23 gears and see what that does for mileage and highway driving. Next step up is a 3.9:1 gearunit I've got, but I think that will kill the mileage again.
I have a 5.7 Hemi in the works for the car, so I'm sure the Hemi will be able to handle the 3.23 gears or even the 2.93 gears much better than the lo-po 318.