tire size vs mpg
 

I have a 71 3/4 ton 4x4 with a 292 and 410 or 411 gears.with the high price of gas I am thinking of gearing and motor options before I start the rebuild . I know a higher gear will inprove mpg .installing taller tires will make it higher geared but this will lower mpg or it did for me on other trucks.I have found nobody who can explan this to me .maybe someone on here knows thanks.

I am just guessing here but the moderate change in final drive ratio that you get with a taller tire is not enough to offset the increased power (read more gas) a larger tire takes to turn.

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.

I will also add to what jeepster said and I agree totally with him, When you add bigger tires your also adding more weight to a vehicle, add to the fact that the front tires being bigger create more drag on the road.

Tall skinny is much better than tall wide. And pump up the pressures to as close to max as you can stand. With the 4:10 you have, I don't think that going too tall is going to hurt that much, but that depends on whether you drive on the open road or in town.
I know on my suburban, I went to a 3.07 gear, and 31" tires, and I now get 18-19 mpg, but that is highway. If I were driving around town, I would choose the lightest, skinniest tire and not worry about size.