Allllrighty. Made the drive from Houston to Austin today on 290. I drive a 98 GXE with 85k miles on it. Right before I left, I changed the oil. I didn't stomp on the accelerator ONCE. I set it on cruise control for most of the trip. I was almost always between 65 and 75. The RPMS were at 2500 for all but 15 minutes of the trip I'd say. Never got past 3000. Is my poor gas mileage to be expected at this RPM or should I take a look at my car's everything?

 

No, that mileage shouldn't be so low unless you had 500lbs of rocks in the trunk. A couple of guesses:

1. low tire pressure.
2. need a tuneup.

 

that is really low. i went on vacation, and i put in 10 gallons after 310 miles of driving

 

I have a '99 5 speed SE and do a lot of highway driving. With the cruise set at 78 mmph I get 28 mpg.
Don't know when you last put in a set of plugs, but that would be the first place I'd start to fix the problem.
Good luck.

 

...Fuel Filter, Plugs, Tire Pressure and Oil.

 

bad knock sensor would give you crappy gas mileage as well