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1 or 2 inch diameter increase, if you can increase the air pressure 20 psi more should still increase mpg. Going from a 30 tall tire to a 36 will usually hurt mpg, but you have to recalibrate the odometer too. I had a 72 Blazer and went from stock 350 w 30 " tall street tires to a 396 and 10" wide 33" tires. Gained 3 mpg.
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