Best gear ratio is determined by the type of work and the tire size.
A general guideline would be the following
3.73:1 with 28 inch tires
4.11:1 with 31 inch tires
4.57:1 with 35 inch tires
These numbers can be tweaked for a bit for more economy or more work.
Run the numbers on a gear calculator to see what works best for you. I have had several different gear ratios and tire combinations on my truck. Less rpm at a given speed does not necessarily translate to better gas mileage. It often can translate to less than stellar performance with no mileage increase. My truck did great with 3.70 gears and 28 inch tires. When I changed the tires to a 31 inch tire, the 3.70 gears weren't enough. The truck just didn't have any guts. Milage increased from about 14 mpg to 14.5 mpg but performance decreased significantly.
