How Can I Improve Chevy Silverado 2500 W/Vortec 6000 Fuel Mileage?

[BradleyR]

We own several Chevy Silverado 2500 with Vortec 6000 gasoline engines. Model years range from 2002 to 2005. We used to get 15 to 17 MPG. Now we get 10 MPG. Any tips on how to improve fuel mileage and get it back into the mid-teens?
While it might be nice to extend the fuel mileage beyond the original, I am simply trying to get it back to where it was when the vehicles were new. We routinely use fuel injector cleaner. Sometimes this improves the mileage but the results usually only last for a few weeks. The results may be deceiving because we often use the cleaner right before a long trip.

These vehicles are usually driven an average of 30 miles per day. Normally these are highway miles with some city driving. Typically 2/3 highway, 1/3 city.
I believe that the intake filter has been changed but I will check it/them. Never thought of the exhaust issue. Sometimes these trucks are driven off road (construction) and it is possible that one or more of the guys hit something.

One of the vehicles (2005 Silverado 2500HD 3/4 TON) has a ladder rack on it. That one never gets good mileage. Usually 10 to 12 MPG. I suspect that the wind resistance of the ladder rack causes the poor mileage.

 

[Dave2]

to loose that much MPG is dramatic, when was it / they last tuned, ever had the intake air filter replaced, these get choked up with dust and grass seeds insect bodies they all get sucked in, this will cause the engine to run a very rich fuel mix, ever checked to see if the vehicle has dragging brakes, not releasing fully,

ever bounced over a rock etc and damaged / squashed the exhaust system, this will restrict the burnt gases and help to choke the engine, hence more fuel consumption, other than bad driving habits, which you should be the best judge of, or is the vehicle being milked of it's fuel by maybe unknown sources, like theft.