I towed with a QC 4x4 2500 V10 and 5 speed manual and always averaged 12.5mpg towing my 11,000 pound 5th wheel trailer (Yellowstone to Los Angeles at least twice a year). Never had any engine problems, but had to change the clutch once. Always towed around 58 - 60 mph on the freeway. At first I towed faster but the mileage was terrible so I learned to keep it slower.