Pull the gas tank and and modify it for larger capacity. Or get a large fuel cell you can put in the back that you have a small pump in that will pump into the larger tank... like in the old trucks when you had to cycle / switch tanks...
I think tom also does economy tunes that may help you squeeze a MPG or two out of each gallon. Can't recall if he has ever gotten data back on how much more it gives you.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)