How to get better gas mileage

You can increase gas mileage up to 10% just by changing your air filter, according to the EPA's FuelEconomy.com site.

The air filter is a piece of thick corrugated paper with a rubber ring around it. And boy, does it get dirty. Like your furnace filter, but your house doesn't move. All that dust and smoke you drive through in your car? It becomes a dark grey or brown coating on your air filter. And that prevents your engine from getting enough clean air.

It's very easy to change the air filter on many cars. Most filters cost $5 to $15, according to the manager of a local AutoZone store. Two screws or a couple of clips, five minutes, and it's replaced.

Inflation due to under-inflation

The National Highway Traffic Safety Administration estimates one-in-four cars and one-in-three light trucks and SUVs has one or more tires under-inflated by 8 pounds per square inch (psi) or more.

The Carnegie Mellon University Sustainable Earth Club studied 81 random vehicles in a parking lot and found that 80 of the 81 had under-inflated tires. The average rate of under-inflation was 20% -- soft tires, indeed.

The EPA estimates that for every 1 psi of under-inflation, fuel economy drops by 0.4%. That's not much, but if the tires are under-inflated by 8 pounds, that's a 3.2% drop in fuel economy. About 1.2 billion gallons of fuel are wasted annually due to under-inflated tires, the NHTSA estimated in 2005.

Hundreds of dollars in savings

So, by changing your air filter and pumping up tires that were under-inflated by 8 pounds, you're likely to get a 13.2% improvement in your fuel mileage.

Take an average U.S. vehicle driven 12,000 miles at 20 miles per gallon. That's 600 gallons of fuel a year. At $3.60 per gallon, that's $2,160 a year.

Increase fuel economy 13.2%, to 22.64 miles per gallon: You use about 530 gallons, at a cost of $1,908 per year, for a savings of $252. Two cars would mean twice the savings. Drive more miles (and I know a lot of you do)? More savings. One or both of the vehicles is a heavy SUV? More savings.

Beyond the savings, properly inflated tires make the car handle better and make it much safer. In fact, that's what the NHTSA really cared about when it did the research.

Proper inflation also makes your tires last longer.

Slowing down saves gas

From: Money.ccn.com

In a typical family sedan, every 10 miles per hour you drive over 60 is like the price of gasoline going up about 54 cents a gallon. That figure will be even higher for less fuel-efficient vehicles that go fewer miles on a gallon to start with.

The reason is as clear as the air around you.

When cruising on the highway, your car will be in its highest gear with the engine humming along at relatively low rpm's. All your car needs to do is maintain its speed by overcoming the combined friction of its own moving parts, the tires on the road surface and, most of all, the air flowing around, over and under it.

Pushing air around actually takes up about 40% of a car's energy at highway speeds, according to Roger Clark, a fuel economy engineer for General Motors (GM, Fortune 500).

Traveling faster makes the job even harder. More air builds up in front of the vehicle, and the low pressure "hole" trailing behind gets bigger, too. Together, these create an increasing suction that tends to pull back harder and harder the faster you drive. The increase is actually exponential, meaning wind resistance rises much more steeply between 70 and 80 mph than it does between 50 and 60.

Every 10 mph faster reduces fuel economy by about 4 mpg, a figure that remains fairly constant regardless of vehicle size, Clark said. (It might seem that a larger vehicle, with more aerodynamic drag, would see more of an impact. But larger vehicles also tend to have larger, more powerful engines that can more easily cope with the added load.)

That's where that 54 cents a gallon estimate comes from. If a car gets 28 mpg at 65 mph, driving it at 75 would drop that to 24 mpg. Fuel costs over 100 miles, for example - estimated at $3.25 a gallon - would increase by $1.93, or the cost of an additional 0.6 gallons of gas. That would be like paying 54 cents a gallon more for each of the 3.6 gallons used at 65 mph. That per-gallon price difference remains constant over any distance.

Engineers at Consumer Reports magazine tested this theory by driving a Toyota Camry sedan and a Mercury Mountaineer SUV at various set cruising speeds on a stretch of flat highway. Driving the Camry at 75 mph instead of 65 dropped fuel economy from 35 mpg to 30. For the Mountaineer, fuel economy dropped from 21 to 18.

Over the course of a 400-mile road trip, the Camry driver would spend about $6.19 more on gas at the higher speed and Mountaineer driver would spend an extra $10.32.

Driving even slower, say 55 mph, could save slightly more gas. In fact, the old national 55 mph speed limit, instituted in 1974, was a response to the period's energy crisis.

It was about more than just high gas prices, though. The crisis of the time involved literal gasoline shortages due to an international embargo. Gas stations were sometimes left with none to sell, and gas sales had to be rationed. The crisis passed, but the national 55 mph speed limit stayed on the books until the law was loosened in the 1980s. It was finally dropped altogether in 1995. (The law stuck around more because of an apparent safety benefit than for fuel saving.)

Despite today's high gas prices, don't expect to see a return to the national 55 mph speed limit. The law was unpopular in its day, and higher speeds have become so institutionalized that even the Environmental Protection Agency's fuel economy test cycle now includes speeds of up to 80 mph.