I don't know what the auto is suppose to get, but it depends on how you drive and your gearing. You could also add a good additive to lubircate your fuel pumps and clean the injectors. My truck gets 23 mpg at 115 km/hr but she has about 1000 lbs extra with the quad deck and bumpers.
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05 3500 6 spd, QC, SB, SRW, 3.73, superchip, K&N cold air, exhuast, 2 " leveling, 315/70/R17 BFG AT, home made bumpers, 15000 lb warn, quad deck.