If you're not gonna use them for the hunting season get the ones that most closely match the factory diameter for your gear ratio. That is what the best fuel economy will typically be.

If you have changed gear ratios then your gonna have to do some math to figure out where you should be. MFG's typically try to meet the best fuel standards to me the regulations set for them. It's a reasonable target to shoot for in the off season for new tires on your truck. Also narrower tires if all else is equal get better fuel economy and less wear and tear on the front drive, wheel bearings, and steering.

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