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My average over about 40,000 miles with a CAT C15 with Allison towing about 6,000 lbs was 5.6 mpg. As others have indicated you can get better mileage on the flats or going downhill but everything combined I was getting only 5.6 mpg and that is driving between 65-70 mph most of the time. I was going back and forth over the rockies quite often too.