I can't think of any worthwhile mod that will improve your mpg sufficiently to save you money. Even an ecconomy based re-map will show miniscule gains in comparison to it's cost. If you manage to save 2-3mpg, you need to do many thousands of miles before you start to see any meaningfull savings.The best method of improving mpg is to adopt a more frugal style of driving as often as you can be bothered.