I found the info that ib was talking about, the one I looked at had a 2012 F350 dually, a 2012 GMC 3500 Dually and a 2011.5 Ram 3500 dually, the Ford and Chevy had DEF, the Ram didn't...below was what was reported at that time. Ford had the best mpg, with GMC close by and Ram, without DEF, "bringing up the rear":)
"Fuel Economy
Our final test of the three heavyweights was fuel economy, because every time you have to stop to refuel, you lose time and money.
We measured fuel consumption over almost 2,000 miles of travel with the trailers behind the trucks the entire time. The results exclude segments where we were testing the trucks, such as on the mountain climbs and at Chrysler’s proving grounds. As we’ve seen in earlier tests, the Ford F-350 had the best fuel economy while towing, at 9.5 mpg. The GMC was close behind at 9.1 mpg, or a difference of $22 over 2,000 miles. The Ram had the worst mileage, at 8.5 mpg, costing $115 more to operate than the Ford."
