The most economical thing to do is get your computer custom tuned.  Check
out http://www.wait4meperformance.com/ he can help you to maximize your
power/mileage.  If you get a "hot" tune, you may have to burn super
unleaded, but can get 3 mpg better, and since all gas is high right now, the
difference between a $45 fill up and a $50 fill up is pretty negligable.  I
bought one more to correct for different size tires and to get better shifts
(remove part of the torque management).  Jesse is by far the least expensive
progammer out there.