seems like your fuel economy is spot on if you are towing in the city. Don't even ask what a gas powered truck would get. My fuel economy is hand calculated - but the overhead display always reads less on my truck. It said 21 mpg but it was actually 23.5. Keep in mind that was driving to Mannyberries and back. Going to 35's lowered my rpm by about 200 on the highway, keeping the engine closer to its sweet spot @ 2000 rpm on the highway (the G56 with 3.73 is equal to the NV5600 with 4.10s) in top gear.
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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.