Installing larger tires will gain ground clearance, increasing the angles of approach and departure angles. Bigger tires also create a more aggressive terrain presence. With increased sidewall size and more air between the tread and wheel, the tire will absorb more shock. A major disadvantage about increasing tire diameter is they don't work well with OEM gearing. Manufacturers build trucks for the street, which means they select high gearing for fast acceleration and better gas mileage. When larger tires are installed, both their increasing size and weight tend to wear out clutches of the transmission sooner, due to First gear being so high. Engine performance and gas mileage will suffer because the engine is working outside of its optimum rpm ranges. Due to the increased tire rollout, distance and revolutions will affect the ring-and-pinion gearset. By changing out the ring-and-pinion gearset when increasing tire diameter from a factory 29-inch diameter to 38-inch diameter, a factory ring-and-pinion setup with 3.73s won't cut it. Stepping up to 4.10s will bring the engine rpm back to the vehicle's sweet spot or proper power and torque band. Most V-8 engines are rated at 2,000-2,600 rpm for highway cruising.
Maintaining proper air pressure in tires is a major factor of the better gas mileage equation. To ensure accurate PSI readings, only check tire pressure when the tire is cold. Under-inflated tires cause rolling resistance and drag, which causes friction that causes heat and accelerated tire pressure, causing a possible blowout. Under-inflated tires will effect gas mileage and tire wear, and will effect 0.04 percent of your miles per gallon for every 1 psi drop in pressure of all four tires. All four tires should have equal air pressure for optimum gas mileage and handling performance. Improperly inflated tires will also show uneven wear patterns and cupping on the outside edges of the tire, due to the tread surface concaving in the center of the footprint. The tire sidewalls will breakdown, due to improper inflation, losing structural support. An over-inflated tire will show wear, down the center of the tread, due to the tread surface convexing in shape.
The chassis alignment is very import to a vehicle's rolling performance. The front and rear axle center lines must be squared to one another. The frontend's alignment should have some toe-out, around 1/16 to 1/8 inch, which allows for straighter tracking down the road. If the frontend is out of alignment with the rearend, or vice-versa, the wheels and tires won't track correctly, causing the vehicle to crab to one side or the other. Frontend camber and caster will cause uneven tire wear, which will also cause drag and rolling resistance.