My truck currently has 131k+ miles on her and over the last four years has been doing quite poorly on gas mileage.
When I bought the truck in January of '02 I got a consistant 16mpg. Starting about 4 years ago and up until two weeks ago I've been at about 12-13mpg.
Over the last two weeks I have averaged 17mpg. The big change I have made over that time is moving to 91 octane non-Ethanol gas. A smaller change was a fresh tuneup and complete fluid change.
Granted it is mostly highway miles but the motor is much happier and the old girl is running better than she did when I first bought her. I have almost immediate throttle response again. Sure the gas costs more but the extra mileage and performance I am getting is more than worth the additional cost. I'm hopeful but that with the Magnaflow exhaust and Volant CAI I'll be adding in the coming weeks that I'll get a little more and break 18 mpg. Even though I'm doing those mods more for performance than mpg. With my heavy foot those may actually cause me to lose some mpg.