2-bbl 318 / 904 & 7-1/4" 3.23 rear axle.
lousy, neglected engine tune / very lean @ highway speeds / worn timing chain;
6-7mpg daily / 17mpg highway

4-bbl 318 / A518 & 8-3/4" 3.55 rear axle.
AFR tuned engine / new timingchain / recurved distributor / better ignition ;
11-12mpg daily / 15mpg highway

65mph = 2200 rpm @ 20" engine vacuum
A518 = OD only, no lockup
Fuel = LPG


IMO, the heavier internals and friction of the A518, and the absense of convertor lockup and lack of cr and strong torque in the engine, dampened my highway mileage, compared to the 904 at least.

But, over the years the car has become much more fun to drive with the 4-speed auto.

At one time I had a 518 with OD and LU installed, which was even more fun, where the Lockup pretty much felt as a short 5th gear.