[JL8Jeff]

I disagree with the slowing down argument. My Silverado sits a little higher than stock with larger tires and I get my best mpg on the highway at 70-75 mph. I've tuned it for the larger tires so I know it's accurate. The stock clutch fan is said to rob at least 1-1.5 mpg and I have the Nelson harness and the LT1 fans but haven't had a chance to install them yet. I have the Diablosport tuner ($275 shipped) and that helped pick up some mpg but I'm usually driving 50 mph and under in my 10 minute drive to work with several traffic lights so I'll never get decent mpg around town. But on the highway you should be able to pull down 20 mpg with a few tricks. I cut back the EGR tube that goes into the TB/intake to allow better airflow and my truck had a CAI when I got it. Headers would probably help but might not be worth the added cost. In the end, I've done the calculations like Scott showed and it wasn't even worth the cost for me to get a moped/scooter/small motorcylce to justify any type of gas savings.

On a side note, have you checked your rear calipers? Mine were no longer floating like they should so I had to get rebuilt calipers for the rear. And the rear parking brake setup is known to drag so that could be causing some loss of mpg. I would definitely get the rear up in the air and check the calipers (unbolt them to verify both sides are moving in and out) and pull the rotors to check the parking brake assembly, I had to rebuild mine because it was all shot.