[zonker]

Greetings,
I'm embarking on a project to make my 1992 Ford Mustang Convertible (2.3 four cylinder) achieve 40mpg, and I have a collection of rear differential / axle assemblies at my disposal... a 3.73 (in the car now), a 3.45, and a 3.08:1 ratio.

Now I know in theory that the taller the numerical ratio, the better FE you will achieve, but taking into consideration usable engine torque and peak engine efficiency, what rpm should I shoot for when travelling at a 65mph freeway cruise speed.

Currently, with the stock automatic trans, while cruising in OD with the torque converter locked I am seeing around 2400rpm with the current 3.73:1 ratio. If i change the ratios to one of the other two selections, I will see 2220 or 1980 rpm.

Would the car be more fuel efficient at the lower rpms, or am i looking at a scenario where the car will be downshifting more, therefore using more gas, since the engine's peak torque (135) occurs at 2600 rpm.