I have an 07 5.9 2500 long bed. It's getting ready to turn over 150K. Excellent truck however it's a 4X4 with 3.73s so I get hand calculated an average of about 17 around town and empty on the fwy about 20. Goes down in the winter due to winter blend fuel. towing my 24' car trailer full I average about 13-14. In Cali on flat roads that goes to 15 average. Being retired military I'm very anal about maintenance so I'm sure that's what has kept this truck going without any major problems.