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Get the most mileage for your fuel dollars Consumer Reports' tests show how to avoid money-wasting driving habits
How you drive your vehicle can have a big impact on fuel economy. That's the key finding of recent real-world fuel-economy tests performed by Consumer Reports' auto engineers. On the highway, driving smoothly and steadily and not carrying items on top of the vehicle are two of the most significant factors. In slower, city-driving conditions, driving with the engine warmed up and driving nonaggressively made the greatest difference.

We conducted a series of tests on two vehicles: a 2005 Toyota Camry sedan with a four-cylinder engine and automatic transmission and a V8-powered 2005 Mercury Mountaineer midsize SUV. We tested the effects of driving aggressively; carrying a car-top storage box; driving at faster and slower speeds on the highway; and driving with a cold engine, underinflated tires, a dirty air cleaner, and with the air conditioning running. Our results show you how to get the most for your fuel dollar.

Of course, the biggest factor in fuel economy is the vehicle. Even when we simultaneously committed a number of fuel-economy faux pas in the Camry, it still got better fuel economy than the Mountaineer did at its best. ConsumerReports.org subscribers can see the models we've tested that have provided the best and worst fuel economy in their classes.


DRIVING TIPS THAT MAKE THE MOST DIFFERENCE

Minimize driving with a cold engine. Engines run most efficiently when they're warm. In our city-driving tests, making multiple short trips and starting the engine from cold reduced fuel economy for both the sedan and SUV. Engines also produce more pollution and wear faster when they're cold. To minimize cold-engine driving, avoid making a lot of separate short trips with a cold engine. Instead, combine short trips into one so that the engine stays warm.

Drive smoothly. Avoid hard acceleration and braking whenever possible. In our tests, frequent bursts of acceleration and braking reduced the Camry's mileage by 2 to 3 mpg and the Mountaineer's by about 1 mpg. The harder you accelerate, the more fuel you use. Unnecessarily hard braking wastes the fuel you use to get up to speed. Drive smoothly and anticipate the movement of traffic. Use your brakes as little as possible, since every time you hit the brakes you are wasting fuel. Once up to speed on the highway, maintain a steady pace in top gear. Smooth acceleration, cornering, and braking not only save fuel but also extend the life of the engine, transmission, brakes, and tires.

Reduce unnecessary drag. At highway speeds, more than 50 percent of engine power goes to overcoming aerodynamic drag. Don't add to that drag by carrying things on top of your vehicle when you don't have to. We installed a large Thule Cascade 1700 car-top carrier on our sedan and SUV. Keep in mind, however, that the effect varies, depending on the model. Driving with the carrier cut 6 mpg from the normally aerodynamic Camry, dropping it from 35 mpg to 29. It only reduced the boxier Mountaineer from 21 mpg to 20. Even driving with empty racks on the car reduces its fuel economy.

Slow down. Aerodynamic drag exponentially increases on the highway the faster you drive. We tested our vehicles' fuel economy at 55, 65, and 75 mph. Driving at 75 mph instead of 65 reduced the Camry's gas mileage from 35 mpg to 30. For the Mountaineer, fuel economy fell from 21 mpg to 18. Slowing down to 55 mph improved the gas mileage by similar margins: The Camry improved to 40 mpg and the Mountaineer to 24 mpg.


IMPORTANT DRIVING TIPS THAT HAVE LITTLE EFFECT ON FUEL ECONOMY

Keep tires inflated. Our tests show that driving on moderately underinflated tires is more of a safety concern than a fuel-economy issue. We set the pressure in all four tires to 10 psi below that recommended by the automaker. This reduced highway fuel economy slightly, by about 1 mpg for the Camry and by a much smaller margin for the Mountaineer. But more importantly, underinflated tires provide much less grip for turning and stopping and run much hotter. Overheated tires wear faster and can lead to a blowout. Check the pressure of your vehicle's tires at least once a month, when the tires are cold. Also check the tires before and after long road trips. The recommended tire pressure is found on a label inside the car--usually in a doorjamb, inside the glove-box lid, or inside the fuel-filler lid.

Keep your air filter clean. According to our tests, driving with a dirty air filter in modern engines doesn't have a significant impact on fuel economy, as it did with older engines. While fuel economy didn't change, however, power output did. Both cars accelerated much more slowly with a dirty air cleaner. We drove both vehicles with their air cleaners restricted and found little difference in gas mileage with either engine. That's because modern engines use computers to precisely control the air/fuel ratio, depending on the amount of air coming in through the filter. Reducing airflow, therefore, caused the engines to automatically reduce the amount of fuel being used.

USING AIR CONDITIONING VS. OPENING THE WINDOWS

Air conditioning places more load on the engine, which can affect fuel economy. But some auto journalists say that opening the windows at highway speeds can affect fuel economy even more by disrupting the vehicle's aerodynamics. In our tests we found that neither makes enough of a difference in fuel economy to worry about. Using air conditioning while driving at 65 mph reduced gas mileage in both vehicles by about 1 mpg-it might make more of a difference if you drive faster. The effect of opening the windows at 65 mph was not even measurable. Because air conditioning can help keep you comfortable and alert and because most modern cars use it to keep windows defrosted, we suggest that the small trade-off in fuel economy for increased safety is worthwhile.

DON'T USE PREMIUM FUEL UNNECESSARILY

If your car specifies regular fuel, don't buy premium under the mistaken belief that your engine will run better. Most cars are designed to run just fine on regular gasoline. Furthermore, many cars that recommend premium fuel also run well on regular. Check your owner's manual to find out if your engine is designed to handle either grade. Think twice about using the more expensive gas even if your owner's manual suggests "for optimum performance use premium." We have found that the differences aren't perceivable during normal driving. However, if your car "pings" or knocks with lower grade fuel, buy premium.


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