towing fuel economy on 2009 Chevrolet Silverado 2500 HD

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no codes,14,000miles,6.7miles per gallon when towing 9,000 pds.15.0 mpg not towing,3.73 gear ratio,4x4
(2) Answers
There is little you can do about towing economy on a gas motor. My F250 gets 6.5 with my 5th wheel (10,000 lbs) and 12 empty I have 4.10 gears which are better for towing than 3.73. Breathing upgrades such as a cold air intake and high flow exhost will net you a little better mialage as will keeping the speed down to 55mph
That sounds about right for a gas engine. Diesel should be closer to 10-18mpg depending on gear, open or closed trailer, altitude, speed, etc. The difference between 60 and 70 MPH can be 5-6 MPG. The 4.10 gear is great for getting started or on hills but worse on MPG because it is higher RPM's for the same speed. 3.73 is best if you drive the truck a lot without towing also. Lower than 3.73 is great for MPG but completely worthless for towing.

Our '09 Sierra 2500HD Diesel gets close to 20mpg not towing and 15-18MPG towing a tractor with total towing weight of about 7000.