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Going too fast—or too slow—can wreck your fuel economy.

Before, Americans might have jabbered about gas mileage, but most drivers cared more about horsepower or cup holders. The era of $4 gas has sure changed that. Suddenly everybody’s a conservationist, desperate to squeeze a few extra miles out of every tank and cut their gas bill.

Thousand of people who can afford to are trading in big cars for smaller ones. But millions of others own SUVs or other large vehicles that they can’t trade in without losing money. Then there are worries about a shaky economy, which makes it a risky time for many to make a big purchase. That leaves plenty of drivers stuck with a car they bought when gas was cheap and fuel economy wasn’t an issue. The only solution: finding ways to boost mileage.

By now, most people know the boring basics: Make sure your tires are properly inflated. Change your oil and air filter regularly. Remove heavy items from your car.

Still, watch the drivers on any highway, and you’ll see fuel being wasted in every lane. That’s because many drivers these days simply don’t know much about how cars work and don’t understand the mechanics of mileage.

So I asked Jack Pokrzywa, manager of ground vehicle standards for SAE International, which sets technical standards for the automotive industry, to help explain what makes gas mileage go up or down. Here are some of the most common mileage mistakes:

Driving too fast. Everybody knows that highway mileage is usually better than city mileage. So the faster, the better, right? Wrong by a mile. Most American cars operate at peak efficiency—generating the most forward momentum with the least amount of fuel—between 50 and 60 miles per hour. There’s nothing magical about that range, except that the government establishes the city and highway mpg ratings for cars by operating them within certain speeds for a short period of time. Automakers want to get the highest mpg ratings possible, so they engineer their cars to be most efficient at the speeds at which the government tests them. If the government tested at 30 miles per hour instead, then no doubt carmakers would engineer their vehicles to be most efficient at 30.

At speeds over 60mph, gas mileage drops off a lot more than most drivers probably realize. “The aerodynamic drag created by a vehicle moving through the air increases exponentially,” says Pokrzywa. It takes more power to overcome the added resistance, which forces the engine to work harder, burning more fuel. A lot more.

If your car has an on-board computer that displays your instant gas mileage, the difference between 60 mph and 80 mph will be obvious—and substantial. At 60, a typical four-cylinder car might average about 30 mpg; at 80, it could fall to about 20 mpg. In other words, your gas mileage going 80 miles an hour on an open road might barely be better than the mileage you get navigating stoplights and city traffic.

As a rule of thumb, the best way to determine your car’s sweet spot is to watch the tachometer, which measures how hard your engine is working, rather than the speedometer. Aim for the lowest rpm in the highest gear, while still having a comfortable degree of power available if you need to pass or maneuver quickly. That will indicate that your car is doing the least amount of work to stay at the speed you’re going.

Driving too slow. By the same logic, driving below your car’s optimal speed is inefficient, too. Some drivers may reason that if driving faster burns more fuel, then driving slower burns less. But that’s only half right. Below your car’s optimal speed, the engine may be doing lots of work that’s accomplishing little, getting less momentum from the fuel burning in all cylinders than if you simply sped up a bit. Besides, Pokrzywa points out, “you can’t go 15 miles per hour on an expressway.”

If you have a manual transmission and know precisely which gear is most efficient in each speed range, you might be able to get close to your car’s optimal performance at speeds below 50 mph. But few drives are that skilled, and most cars today have automatic transmissions, which are likely to downshift into a lower gear if you’re dragging along too slowly—which itself can burn excess fuel.

On local roads, obviously you can’t tear around at 55 (or at least you shouldn’t), so you have to accept a degree of inefficiency. But the rpm principle still applies: Try to achieve the lowest rpm within a speed range that’s safe and appropriate for the roads you’re on.

Speeding up too quickly. Ever notice how your cruise control increases your car’s speed when you push the “accel” button? Usually, very gradually. That’s because the car’s computers know that gaining speed slowly allows the engine to operate most efficiently. Humans, by contrast, tend to want immediate results. So we floor the gas, flood the cylinders with that precious fuel, and burn up a lot more of it than needed. Mileage experts recommend using cruise control as much as possible because it’s more rational than humans. And it’s smart to mimic that driving style even when cruise control is turned off.

Braking too much. Every time you press the brakes in an ordinary car, you lose a lot of energy. Of course, the brakes are there to prevent you from running into stuff, and usually there’s no choice but to use them. Which means the real mistake most people make is going too fast in the first place, requiring more braking than necessary.

Practically everybody does this: You take off from a stop sign and speed up to 30 or 40 mph, then suddenly you have to slow down to avoid rear-ending a car ahead that’s going slower. If you think about it, it’s a foolish way to drive: Speeding up and then hitting the brakes, over and over, doesn’t get you there faster. It’s just a herky-jerky habit, borne of impatience. Driving more gently will get you there just as fast, usually, while saving money—and maybe even help prevent an accident.

Idling. Your car is least efficient when it’s going nowhere–your mileage, effectively, is zero. Yet the car still burns fuel if it’s turned on. Some “hypermilers” like to turn off their cars when stuck in traffic or sitting at a red light. That’s basically what hybrids are programmed to do automatically.

Of course, it’s not really the safest idea to turn off your car on an active roadway, so don’t feel obligated. But what about when you’re waiting by the curb for somebody to emerge from their house? Or sitting in a parking lot? Or lazing in your own driveway in the morning, absent-mindedly sipping your coffee? If you want to run the radio or air conditioner, use the accessory setting by turning the key half-way. But do yourself a favor and run the car itself only when you need to. It’s not like gasoline grows on trees.

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