I have a 96 f250 mostly stock with 4.10 gears an intake and cat delete. I normally get 13 mpg around town empty with a light foot but today i hauled 2 1/2 tons of hay in 2 trips. when i pulled in to fill up with diesel, i topped it off the same amount i normally do, and did the math. 129 miles and it took 8.053 gallons to fill it. am i wrong or is that 16 miles per gallon? this was in town mostly with speeds less than 50 mph and overdrive off..... any ideas of why the better mileage while towing?