[Big Dave]

Having done the gear swap thing (3.73 to 3.08) I can tell you it is effective. I gained 3 MPG from doing it. But I have a diesel known for its huge low-end torque.

Before you do a gear swap, I'd recommend a long talk with an automatic transmission expert. A numerically lower rear end will slow down your torque converter and reduce ATF flow through the cooler, attenuating cooling. In a pickup, this leads to spectacular transmission failures - parts and fluid scattered down the road. Been there, done that.