The Ford F-350 diesel averages about 20 miles per gallon on the highway, depending on the amount of weight the truck is towing. Diesel engines get better mileage than gas engines, which is why the V-8 engine in the heavy-duty F-350 can still get in the 20s on the highway. By changing your driving style, maintaining your truck and making minor modifications to the truck itself, you can improve your mileage.
Stay current on the Ford maintenance schedule. Click on the link "Ford Maintenance Schedule" under "Resources" on this page to search for your truck's maintenance time frame. If you are doing heavy-duty towing or hauling, you will need to get your truck checked more often. Keeping your engine running well and changing fluids regularly, including oil changes, is one of the best ways to improve mileage.
Replace the stock air filter with a larger and higher quality air filter. Greater air flow can improve your fuel economy.
Remove excess weight from the back of the truck whenever possible. The F-350 diesel is already a heavy truck and added weight only makes the engine work harder. If you do not need to haul material or tow a trailer, remove it from the truck.
Change your driving style. Accelerate slowly and anticipate stops rather than slamming on the brakes. If possible, try to avoid city driving, since higher mileage is achieved on the highway. Avoid driving during peak traffic times when you are forced to idle on the freeway. Idling burns gas but does not get you far.
Disable the backpressure valve on the turbo, if your F-350 is a turbo diesel. The backpressure valve can cause your mileage to be lower, especially in cold weather.
