Everybody knows that cars run on gas; what people often forget, though, is that they also run on air. With that in mind, some readers noted that K&N high-flow air filters can dramatically improve efficiency. "DVilla4940," put the improvement in numbers, claiming that a K&N "will give you about 2 more miles per gallon."
For those who are more mechanically inclined, a customized air intake and exhaust system can also have big dividends at the pump. "Ron," claimed that installing a "cold air intake," adding "dual exhaust with flow-master mufflers" and using "the lightest-weight oil your engine can stand" will all add miles per gallon to a tank.