[argonaut]

Snow,
Logic would tell you that a numerically lower gear, and larger diameter tires would result in lower engine rpms at a given vehicle speed, and thus better fuel mileage. However if you gear it so that the engine is spinning an an rpm that is too low on the horsepower curve it will actually consume more fuel, because it is making power far less efficiently than at a higher rpm. Each motor has a sweet spot that is optimum for power and economy at sustained operation. This is what the engineers determined when they designed out trucks.
In general, smaller displacement motors make power at higher rpms, than large displacement motors. so the gearing is usually numerically higher to put the motor in the sweet spot. So sometimes, when you've mixed and matched engines, transmissions, rear axles, and have a big heavy vehicle, it can be beneficial to numerically increase the gear ratio to get better power and better fuel efficiency.

An example would be my F0rd Bronco. I had 33" tires on it with 3.55:1 diff gears and overdrive with a small block. I recent put 31" tires (close to stock size) back on it, and my fuel mileage has gone up by at over 1mpg. When you're getting 13mpg, that translates into an 8% increase. If I want to run 33" tires again, I really should install 3.73:1 gears in the diffs. Make sense?