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A higher gear doesn't always give better mileage. As with everything there will come a point where you've gone too far. Too low an RPM and the engine will be lugging and not be at it's most efficient.
Tire and gear are going to combine for your final drive, either one can affect your final drive, and mileage for better or for worse.
Final drive aside, your tire flattens and bulges where it meets the road, The treads squirm and flex as the tire goes round. Bigger tires do more squirming and flexing than little tires. The energy that goes into flexing the tire is energy robbed from moving forward, so a bigger tire is going to rob more energy.
I think the energy robbed by the big tire equalizes out with the gains from higher final drive that it gives, so if your gears are not too high already you may not get much net loss from the tire size.
Increased air pressure stiffens up your tire and can make a big difference in mileage.
On your 3/4 ton truck, switching to a taller 16" highway tread truck tire, can boost your final drive, while maintaining a low rolling resistance.
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