The 1988 Chrysler 5th Ave was rated at 23 mpg highway by the EPA. These had the 318, lock up torque converters, and often had 2.2 or 2.45 axle ratios. The 5th avenue will probably be heavier than most A bodies.

The 5th avenues were NOT noted for performance! The high rear axle ratio was one of the primary reasons for their lackluster performance. An overdrive transmission will allow for the low RPM's needed for highway fuel economy, but the lower rear gears will allow for more acceptable performance.

If a person's primary interest is economy, a 2.76 with an overdrive could potentially yield better mpg's than a 3.23 gear in many situations. The 3.23 would definitely yield better performance and probably wouldn't hurt mileage to much. Of course this will depend on where and how the A-body in question is being driven. For example, in an area with lots of steep hills, a 2.76 gear may not allow an overdrive transmission to shift into overdrive very often. In this situation, a 3.23 with overdrive would obviously yield better mileage, assuming the transmission can shift into overdrive and have the necessary power to pull the hills. (and the 3.23 gear would hold the car back better on down hill grades)