2.80's--4.11's gas milage?

[Jeff65]

20 or above is considered good. 15-20 marginal and less than 15 poor.

Typical cars in factory configuration and in good condition and state of tune get 18 mpg city and 20 mpg on the highway.

Cars with numerically high rear axle ratios will get lower gas mileage due to higher engine rpms at highway speeds. It isn't directly proportional but approaches it. Hence if you have 3:50s instead of 3:00's the mileage expectation would be:

3:00/3:50 x 18 and 3:00/3:50 x 20 => 15.4 city and 17.1 highway mpg respectively.

Compare this to a 5 speed, then the numbers are something like:

3:00/3:50 x 18 and 3:00/3:50 x 20 x 1/0.68 = 15.4 and 25.2

The reason for this is the overdrive gear in the 5 speed provide a lot better highway mileage. In actuality, a carbed engine car will not get 25.2 mpg on the highway but will get 22.5. An EFI engine will get the 25.2 mpg.