7 Gas Saving Myths Debunked

MHUB - October 8, 2014

Myth No. 1: Cold-Weather Fill-er-Ups

The Lie: "Make sure to top off your tank first thing in the morning when the fuel is cool for more density and energy!"

The Truth: Yes, cooler fuel is denser and, technically, will have more energy per gallon. Not that your car cares, because it automatically corrects the mixture ratio so the proportion of air-to-fuel is chemically correct. And it doesn't matter whether fuel is below freezing or so warm that it almost boils in the fuel rail—your car will compensate. So why should we care how dense the fuel is when it comes out of the pump? We don't buy fuel by its energy content—we buy it by volume. In theory, then, we could get more for our money if the fuel is cool.

So the myth is partly right, but there's virtually no difference in the temperature of fuel stored in underground tanks as the day progresses. The tanks are buried deep enough to keep the temperature constant, although there are seasonal changes. What's more, this myth ignores the fact that most modern fuel pumps are temperature-compensated to equalize out any density difference.

Myth No. 2: The Clean Air-Filter Swap

The Lie: "If there isn't enough air, a dirty cleaner will make your engine burn more fuel!"

The Truth: Back when cars had carburetors, changing your air filter could have helped. Today, the on-board computer injects exactly the amount of gas your car needs. A small restriction in the air flow won't make the engine run too rich. The pleated-paper air filters used today are very large, and can hold a lot more dirt than you might think before they start to choke the engine. An oily filter—the result of blow-by fumes from the crankcase—can make a filter plug up pretty fast. Modern crankcase ventilation systems are far less prone to oil them up than they used to be.

Still, that filter probably won't affect mileage until it's so plugged that it makes the Check Engine light come on. But if you have an older car or truck, or any equipment that still uses a carburetor, it is important to keep an eye on the air filter, to make sure its free-flowing.

Myth No. 3: Going Premium

The Lie: "More expensive fuel offer better fuel economy, because it's higher quality!"

The Truth: The media has confused this issue terribly. Post-1996 model-year cars virtually all have knock sensors. If the octane rating is too low, the computer will roll back the ignition timing a few degrees to compensate. This will reduce peak engine power, and also increase fuel consumption. So, in some sense, this myth gets it right—if your car is supposed to run premium. One of our long-term test cars in the PM fleet delivers an impressive 25-percent better fuel economy running on premium than it does on regular, although our test was hardly done under rigorous conditions. I'll leave the math for an exercise, but I calculate the price differential between 87- and 91-octane fuel at more like 6 percent—at least in my neighborhood. Your mileage may vary, but it's worth trying to see what happens in your car. If you top 6-percent better fuel mileage on premium, it may save you money to run it.

Of course, older cars, built in the days before knock sensors, may be damaged by running too poor a grade of fuel. Spark knock can actually burn holes in pistons, so burn substandard fuel in non-knock-sensor-equipped cars at your peril.

But wait, there's more! Is your car supposed to run on Regular? There are more BTUs (energy) in regular than in higher grades. You may very well get better miles-per-gallon from regular.

Myth No. 4: Tire Inflation

The Lie: "Hey, hybrid owners and hypermilers: Overinflating your tires will reduce rolling friction and increase economy!"

The Truth: Underinflated tires do have more rolling friction and waste gas. They also have less traction and stability because the low pressures don't stabilize the tire carcass, letting it squirm around. There is also a lot more internal friction in an underinflated tire, and that wastes gas. Overinflated tires also have less traction because the tire contact patch gets proportionally smaller as the pressure goes up. I say keep the pressures where the car manufacturer suggests. As always, check the owner's manual and follow tire-pressure recommendations.

Myth No. 5: Cutting the A/C

The Lie: "Leave off your air conditioner and open your windows, and you'll get more MPGs!" (or: "Using your A/C on hot days actually uses less fuel, because your aerodynamics are better with the windows up!")

The Truth: The aerodynamic drag of any vehicle is proportional to the square of its speed. But the fuel consumption is proportional to the cube of its speed. At low speeds around town, the 5 to 8 hp necessary to run the air conditioner will decrease fuel economy measurably, while the small aero drag from opening the windows is negligible. Pull out onto the freeway and increase speed for 35 to 70, and the numbers reverse. Well, actually, the A/C consumes exactly the same amount of power—but the extra fuel required to run with the windows open goes up by a factor of eight.

Either way, when you come back to the car after parking in the sun, open all the windows for a couple of minutes until the sauna inside cools off. That way the A/C won't be working so hard to cool off things initially. After your car's interior is closer to equilibrium with the outdoors, it won't take as much power—or gas—to keep cool.

Some hybrids have very efficient, electrically-powered A/C compressors, and usually have highly optimized aerodynamics. So their low power consumption and higher aero losses, percentage-wise, make the break-even point at a lower speed.

Myth No. 6: The Gadget Con

The Lie: "Just add this magnet to your fuel line, or this high-tech whirligig to your intake, or pour this magic dust into your tank, and enjoy a 30 to three hundred percent bump in fuel economy!"

The Truth: Rubbish, I say. I've tested dozens of these things. The Environmental Protection Agency has tested hundreds. Trust me, if a $10 gadget could improve fuel economy by any significant amount, the car company that used it would have a crushing advantage in the world marketplace. And they don't, because that stuff simply doesn't work. You can't run your car on water. Most of these

Myth No. 7: Short-Stop Idling

The Lie: "Keep the engine running while you pick up the mail and a cup of java, and you're save gas fast!"

The Truth: Your car generates negative miles per gallon if it's running and not moving forward. It's burning fuel— enough, in fact, to substantially lower your economy even when idling at traffic lights or in slow traffic. On the other hand, once the engine is warmed up, it uses no fuel whatsoever to shut down or restart when you need it. None. That's why an integral part of the fuel-saving strategy of any hybrid is to turn off the engine completely when the car isn't moving, or moving slowly. Don't leave your car idle, even for 30 seconds.