I am currently using Geolander P265/70R16's HTS, can I use something else to improve the gas mileage?

Michelin LTX is definately the best tire for trucks and suv's for ride quality, gas mileage, and noise. It is a bit more money but you will probably see 100 to 150,000 miles from them.

The less aggressive the tread pattern, the better the mileage.

One thing that WILL affect your mileage is by putting on oversize tires, this will cause a drastic reduction in perceived mileage. Taller tires mean less rotations per mile, also shows less miles on your Odometer. A good tire dealer can show you how a calculation that will compensate for the size difference.

For Example, my Chevy Silverado has larger tires on, but I have to add 3% to the mileage to accurately figure my gas mileage...

Yamaha tires are the best to use