in the late 60's, where my dad worked, one guy bought a new VW beetle. the guy was very frugal, and bought the bug mostly for the gas mileage.

every afternoon, when the guy was busy, they would add a coke bottle or two of gas to the bug.

after several weeks of this, they kept asking the guy about his mileage in the bug. the guy would just shake his head and report that he was not able to really even check because the damn thing STILL had over a half tank even though he had been driving it for nearly 3 weeks!

they kept this up for several more weeks (gas was cheap then!)

the guy probably got better than 75MPG for a couple of months! :p