Gas Saving Tips: Common Fallacies

Most tips you hear about saving gas are true. Changing your oil, keeping your tires properly inflated and removing excess weight are all good things and will save gas. Some gas-saving tips that appear sound on the surface are really wrong. What may appear to be common sense does not necessarily hold up when the underlying mechanics are investigated. Other advise you have been given is just foolish. Here are a few common misconceptions about saving gas.

The pickup truck and the tailgate
False: Driving with the tailgate down will make your pickup truck more aerodynamic and improve your gas mileage.

The position of the tailgate has a nominal effect on a truck's gas mileage. Pickup trucks are designed to be their most aerodynamic with the tailgate up and locked. This topic created a storm on the NPR show, Car Talk, eliciting wind tunnel engineers and one former General Motors president, Bob Stempel, to set the record straight. Even the Discovery Channel took on this myth on the show Mythbusters. Conclusion? Myth busted.

The air conditioner
False: Driving with your air conditioner on decreases you mileage.

A car's air compressor does pull power from the engine using some gas, but the effect is negligible on late model cars. To save that small amount of fuel, you will need to turn the A/C completely off. Before you suffer the heat or roll down the windows consider that on most cars the drag increases when the windows are down. The net effect is that the increased drag cancels out any fuel savings from turning the A/C off. Use your A/C and be comfortable on hot days.

Park in the shade
False: Parking in the shade or using window screens will make the air conditioner run less and save fuel.

The nominal fuel used by the air conditioning system (see above) is used by the load the compressor puts on the engine. On most cars, if the A/C is on, the compressor is engaged. The only way to disengage the compressor is turn the A/C off. If someone told you this one, they don't have a clue about how a car engine works. Regardless of how fast the fan is blowing or how low the thermostat is set, the belt connected to the compressor is turning. Of course, a shady spot or windshield shade does reduce the temperature, but the only effect is on human comfort, not fuel economy.

The oil flush
False: Back flushing your oil on a regular basis improves mileage.

Changing your oil and filter on a regular basis is one of the best things you can do to keep you car running efficiently for a very long time. If you change your oil every 3000 to 5000 miles, it will never be necessary to flush the system. Unless you are restoring an old engine, save your money on what is fast becoming one of the biggest scams around. Next time your quick-change oil shop recommends an oil flush, pass.

The half tank
False: Never let your gas tank get less than half full because you get better gas mileage out of the top half of the tank.

It only seems that way. Your gas gauge moves further, faster as the tank gets lower because most tanks are irregularly shaped. Your fuel economy has not changed. It is merely a mechanical, optical illusion. Gas gauges are not meant to be scientific devices, just good estimators. By the time your gauge reads half a tank, you've used more than that. Logically, you get better gas mileage with less gas because your load is lighter.

The wax myth
False: A good wash and high-gloss wax job will reduce wind resistance.

Who ever told you this doesn't understand aerodynamics or fluid mechanics. The wind doesn't give two hoots about your wax job. A good wash can improve safety by clearing windows, but the only thing wax does is protect your paint and feed your ego.

Headlights
False: Headlights use fuel. Don't drive with your headlights on during the day.

Sure all power, including electrical, in a car is ultimately derived from gas, but headlights don't burn gas. The alternator is going to charge the battery regardless of whether the headlights are on or not. This goes hand-in-hand with the A/C myth. Statistics show that cars using headlights during the day are less likely to be involved in an accident. Turn on your lights, be visible, and stay safe.

High octane
False: You get better mileage from higher octane fuels.

The recommended octane level for most cars is 87. Putting higher grades into your Granada or Gremlin does not give you any more miles per gallon. NASCAR and Formula One racers use ridiculously high octane fuel, but they're not getting stellar mileage. Likewise, gas with "special detergents" does little to change your mileage. Gas is virtually the same whether you are filling up at a named-band station or the discount gas mart.

How to calculate your mileage
Determining your gas mileage is very simple if you plan ahead. When you fill up (it is important to do this with a full tank) reset your tripometer to zero. The tripometer is a re-usable odometer. Some cars have a mechanical one that can be reset by pressing a stick/button near the readout. Newer cars have digital tripometers that are normally reset by pressing and holding a button.

The next time you fill up, note the mileage on the tripometer and the amount of gas you buy. If you don't carry a calculator with you, it's handy to just write the miles down on the gas receipt. Don't forget to reset your tripometer for the next fill up.

The formula is very simple. You are looking for miles per gallon, so simply divide the miles from your tripometer by the amount of gas you bought. For example, 191 miles / 9.2 gallons = 20.76 mpg.

The single thing you can do to lessen your gasoline usage is plan your trips. Just leaving 5 or 10 minutes earlier for work can mean the difference between idling in long lines at traffic lights and cruising unimpeded through the streets of your town. Combining errands means fewer miles traveled and more money in your pocket. The next time someone offters free gas saving tips, remember, you get what you pay for.