Adjust the parking brake and check the calipers for no sticking pads and then give it a rest if you have no other symptoms.
24 mpg at nearly 80 mph isn't bad. You are in a sensitive area where more speed just guzzles gas.
http://en.wikipedia.org/wiki/File:Fuel_economy_vs_speed_1997.png
Go try a controlled run at 70 mph or less and you'll be pleasantly surprised.
Plus, don't forget it's hot as the Nevada desert in the Nevada desert, which is bad for efficiency.