Try adding about 10% Diesel. I say about 10% because you will need to play with it a bit to get the best MPG. For what ever reason this process varies from vehicle to vehicle. This increases toy MPG because diesel typically releases about 38.6 MJ/l (138,700 BTU/US gal), whereas gasoline releases 34.9 MJ/l (125,000 BTU/US gal), 10% less (http://en.wikipedia.org/wiki/Diesel_fuel
) or diesel burns hotter than gas. Adding the 10% will not effect or damage your motor either like some potions and elixirs being sold will. I went from an average of 14 MPG to 18 MPG just by adding the diesel for the past 30K with no problem.