Wow, that's a lot of hwy miles.
I concur with the general theme that
changing your driving style/habits will translate into the most immediate MPG improvements. Scangauge (or some form of instant MPG feedback) can help.
My sig has the 5 basic areas you can look into to improve upon:
DRIVING STYLE
- what everyone has already said. Make sure to read the 100+ tips on the upper right corner of the page.
- know your car and what it is capable of (automatic vs manual transmission, is it flat-towable, understand your shift points, optimal speed and rpm)
AERODYNAMICS
- smooth out the air your car has to move through with grill blocks, removing drag-producing objects and surfaces such as roof racks, covering over "unsmooth" features, belly pan, front/rear wheel skirts, smooth hubcaps, front/side/rear wheel fairings, antenna removal, passenger side mirror removal, kammback/boattail
TIRES
- increasing tire pressure up to but not more than the max sidewall rating on the tire. I have my tires up to 40psi.
- switching from stock tires to low rolling resistance tires
WEIGHT
- reduce excess mass in and on the car.
- replacing heavier stock parts for lighter replacements
ENGINE
- make sure it is properly maintained as per manufacturer recommendations from oil, oil filter, fluids, tire rotation/alignment, batteries, spark plugs.