To most, the F-350 seems like the least-likely vehicle in the world to wind up in a discussion about fuel economy. At some point, the numbers get so low, and the differences so minor, that it's easy to just write off fuel consumption as "awful." Those who regularly do use these trucks for their intended purpose, though, know that it's the truck's very thirsty nature that makes every mpg that much more important.
All else being equal -- especially engine displacement -- diesel will always return better fuel economy than gasoline. Diesel fuel itself contains 13 percent more energy than gasoline, and spark-ignited gas engines are typically about 10 to 15 percent less energy efficient than compression-ignition diesels. As of 2013, Ford's advertising department says that this year's gas engine gets 15 percent better fuel economy than any other truck in its class; but, predictably, Ford still doesn't advertise the actual numbers. However, the company does advertise fuel economy numbers for the E350 van.
Drivers of 2012 F350 diesel trucks report about 10 to 12 mpg city and to 15 to 16 highway mpg -- that's slightly better than the 11 city and 15 highway mpg reported for gas-powered E350 vans. That's especially relevant when you consider that the van is about 1,300 pounds lighter and uses a smaller 5.4-liter engine. Additionally, mathematical efficiency models suggest that you should expect no better than 9 city and 13 to 14 mpg for the heavy truck and its larger 6.2-liter engine. Either way you look at it, the 6.7-liter diesel has a clear advantage in fuel economy. However, you're going to have to put in a lot of miles to offset the diesel engine's $8,000 buy-in premium and the extra cost of diesel fuel over gasoline.