Other than preforming scheduled maitenace, the only items that have improved mpg has been oil and air. The change from dino to synthetic improved approximately 1/2 mpg. (that was the orginal reason for GM installing it in the vette) With my 17 inch oem GYs I run 44 psi in the front and 40 psi in rear when empty. At 47,000 miles they have worn even and I still have 5-6/32s left. This provided a full 1 mpg increase due to the reduced rolling resistence.

The EPA tests most of these fuel savings devices and found most to be worthless with some actually reducing performance and a few showing marginal benefits. You gotta believe if any of the auto manufactures could spent a $100 or two per truck and increase the EPA mileage that much, they would in a heart beat.