OTHER GREAT TIPS

Use the correct grade of motor oil

 The grade of the oil pretty much tells us about the viscosity. If you use the wrong grade, you may increase the friction in your engine. It gets hotter and uses more gas.

Air filters

 Replace your air filter when you need to, or your mix won't be right. However don't replace it every time the mechanics tell you to (they make money with it). Try to find out how often you need a new filter.

Turn off the lights

24. Well, this one might be a safety concern. Many Rental Car companies have daylights enabled on their cars which are rather efficient. However every electrical equipment is powered from the alternator which will increase its load on the engine to produce more power. So when you can do it safely, turn off those headlights.

Drive less

Here comes the discussion we don't like to read as much.

Carpool

 Well, if two people are riding in a car, the gas used per person is immediately cut into half. If 4 people are sharing a ride, their individual gas bill becomes only one third. Since they can now use carpool lanes, they won't have to spend as much time in traffic (idle engines use gas too) and get an even better gas mileage, plus they get home sooner. Its not always feasible though.

Combine Trips

 Try to combine trips. If you live outside of town, try to go into town only once and get everything you need done.

Fuel Efficient Cars

If you are in the market for a new car, you definitely should consider fuel efficiencies. However I wouldn't buy a hybrid just for the better fuel economy if I weren't in the market for a new car. You can easily calculate how much money you would save a year and weigh that against the cost of the car (plus the potentially higher maintenance cost).
  Its not commonly known, but Diesel engines can give you a much better gas mileage than Hybrids on long distance drives. That's one of the reasons, hybrids aren't popular in Europe but Diesels are.

Don't fill up unless you are on empty, since all this gas weighs a lot and as we have already learned (force equals .... - you know it). However I usually only do this when gas prices are falling or constant (so I can wait longer and get cheaper gas). When gas prices are rising, I fill up sooner. Due to the psychological impact of rising gas prices, people usually do the exact opposite, which will eventually cost them more.
  The list still grows. Someone made an excellent suggestion for trucks in the forum. Dropping the tailgate of his small truck, Ronald is able to improve the aerodynamics and thus get a better gas mileage. Sounds plausible.
  I also noticed that the first gas station you encounter after a long thirsty stretch is almost never the cheapest. After you leave a national park and didn't have a chance to pump gas for a long time, the first station will often have slightly higher prices as everyone pulls over to fill up as soon as they can. I usually drive a little further to find a cheaper station.
 I do not fill up on gas stations visible from the Interstate I am driving on or from the main freeways. I usually take an exit at a moderately sized city when I am on a road trip and head for the city center. Before I actually get there, I usually find much better deals. Location and Competition are the factors that determine local price fluctations. This means you need to look for places with lots of competition (hence moderately sized city) and avoid prime locations (get away from the main roads). Large cities often have higher gas prices and are harder to navigate. On my last trip, I pulled over whenever I saw a station that had significantly lower prices then I had seen previously, even if my tank was only half-empty. On road trips, the rules are slightly different. Fill up whenever and wherever it is cheap and do not wait for yout tank to be empty.