Diesel truck MPG is typically going to be three to five miles per gallon better than gasoline powered engines of the same size, and as much as ten miles per gallon better than gasoline powered engines with similar horsepower and torque numbers. Diesel is an overall much more efficient fuel for powering a vehicle than gasoline is. Your diesel powered sedan is no different. When compared to engines with similar displacements, diesel engines will get three to five miles per gallon more. Your diesel powered truck, because of the fact that it is heavier and geared for towing and hauling won’t get fuel mileage as high as a sedan will, however.
For more info, see Info on Diesel Truck Maintenance