guys, I hate to disagree, but I actually posted an article from Truck Test digest a while back and the upshot is that the axle ratio has VERY little to do with mileage at cruising speed, and virtually zero at city driving.
The summary is that first of all, the EPA fuel estimates are tested at 48mph!
To quote them:
The reason is aerodynamic drag from the truck’s frontal area, not the gear ratio, is the major contributing factor in fuel economy.
According to the EPA, during the highway test cycle where the average speed is 48 mph, 54% of the engine’s power is used to overcome aerodynamic drag. Drive faster and the engine has to work even harder to push through the air and consumes more fuel in doing so.
“The best fuel economy for the typical truck or SUV is cruising right around 40mph. The EPA test cycle for the highway fuel economy number averages 48mph with a top speed of 60mph,” explains Clark.
“A good example of how drag affects fuel economy is a truck that has a 21mpg highway EPA number. Drive at a 10-percent higher average speed (53mph) and drag causes fuel economy to fall about 1.5mpg. Average 60mph and mileage will drop another 1.5mpg. Run just above70mph and now fuel economy is less than 14mpg instead of 21mpg.”
Anyway, here's the article, from Truck Test Digest and I hope it helps,
http://www.trucktestdigest.com/axle%20ratios.htm
anthony