The final engine in our list is the 6.6L Duramax turbo diesel
. This engine uses diesel fuel in conjunction with a turbo to produce similar power numbers as its gasoline cousins. However, this engine has an extremely high amount of horsepower while still getting great gas mileage. Its gas mileage numbers are around 15 miles per gallon on the highway and 12 miles per gallon in the city. When you take into consideration the 340 horsepower and 400 ft lbs of torque it becomes clear that this engine is extremely efficient. This is the perfect engine for someone who needs to tow or haul a lot of weight or carry a lot of passengers.