It will get slightly worse from where you are now. I run 305/55/20 Nitto Trail Grapplers and get 16-18 mixed but more highway. At 70-75 mph, I get about 17.5 mpg. At 65, I get 18+
I see several posts stating that bigger tires kill fuel economy. That must be geared more toward Mud/Off road tires than what I am running. My truck has 305/40/r22s so I'm wider and run a bigger wheel than stock but my fuel economy hasn't suffered. I think most folks here equate bigger tires with Off-Road tires which are gonna hit your fuel economy on any truck.
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