[HummerJim]

The problem sounds like your odometer is reading incorrectly with the larger tires. They make compensation devices for that, but they are expensive and not worth the money unless you are running monster truck tires. Also, with larger tires you in essence numerically lower your axle ratio, which can help on the highway, but hurt mpgs under lugging condtions.
As for the superchip, they just work by recurving the various engine O2, fuel, timing, injector duration etc. The factory uses a balance between power and durability. The after market chips all lean towards power and away from durability. I had one in my 1988 Corvette and a year afterwards a hole appeared in the #1 piston. When they tore down the engine they found several burned valves, but the power was definitely increased while it lasted.