There are a lot of loaded questions here:
1 - Fuel economy is affected by too many things, such as how you drive. Most guys are getting 22-26 mpg on the high way, though.
2 - Some mods can help a little with fuel economy, such as Cold Air Induction and under drive pullies.
3 - Chips/Programmers increase HP by advancing timing. Thus, requiring expensive high octane gas. BUT, some chips/programmers give you an 87 Octane tune as a choice. This tune might help a tad with fuel economy as some programs allow your engine to run cooler by turning on your fan sooner. The $200 - $400 spent on the chip/programer would take a long time to pay for itself I think if your sole mission is furl economy.
Best way to increase fuel economy is to keep rpms down low and drive less.
