Larger tires gave me an extra 5-7 gallons per tank.. or to be more specific, TALLER tires gives me results. I just have to be careful with my speed until I change out the speedometer gear because I am actually going faster than what the needle reads.

So technically a taller tire gets me a further distance for every revolution of the wheels which adds up after a few hundred miles. I figure that going from the 215-75-15's to the 235-75's adds another 10-12 mph to my actual speed which translates to the higher distance/mileage per tank.