[Big Dave]

The guys have you onto some good and inexpensive ideas, but I'm afraid that sooner or later you will have to change your axle ratio.

Vehicles that don't seem to be sensitive to other things are often way over-geared. A guy I know with a Chevy dualley with a 454 and 4.56 gears got 8 MPG whether he was pulling his 32 foot RV or running light. That's a heckuva difference to not feel. The closer you get to being geared right the more sensitive (in both positive and negative ways) your car will get.

Figure out what axle you have and hit Randy's Ring & Pinion to see what gears you can get.