With gas prices at $4 a gallon, there are a lot of theories about how to increase your mileage. Here's a look at some tips, and whether they are true.
Will turning off the AC increase my gas mileage?
This theory, and its opposite — driving with the windows down increases drag, reducing mileage — sound good but Consumer Reports magazine found it didn't matter in newer cars, especially at highway speeds. "Using air conditioning while driving at 65 mph reduced the (test car Toyota) Camry's gas mileage by about 1 mpg. The effect of opening the windows at 65 mph was not even measurable" they report in the June issue. The difference may be more noticeable in stop-and-go traffic. But Steve Mazor, manager of the Automobile Club of Southern California's research center, guessed it "could be a 5 percent mileage saving." There is likely to be a savings in older cars and the U.S. Energy Department suggests limiting AC use.
Does a clean air filter help?
The Consumer Reports team used duct tape to partially cover their car's air intake, simulating a dirty filter. "We were surprised to find out it didn't much matter," spokesman Douglas Love said. "Onboard computers that adjust the fuel mixtures on recent cars did a surprisingly effective job." The California auto club got similar results. Again, it may make a difference in older models and the Energy Department recommends it.
Do additives or gadgets help?
Mazor said he's helped test about 50 products. Consumer Reports tried devices ranging from $50 to $250. None worked well enough to be worth buying.
Does a warmed-up engine run more efficiently?
Yes. In Consumer Reports' city-driving tests, making several short trips and starting the engine from cold each time reduced fuel economy by almost 4 mpg. Combine short trips so the engine stays warm.
How about idling?
The Car Care Council found excessive idling can use almost 19 percent more gas than turning the car off. When Consumer Reports let their V8 Buick idle for 10 minutes while warming up, it burned about an eighth of a gallon of gas. They suggest turning off your engine if you sit for more than 30 seconds. Others suggest no more than a few minutes.
What does work?
Changing your driving habits, or your car. The best way to use less fuel is to buy a car that gets better gas mileage. But you can also help yourself. Slow down, avoid quick acceleration. On the highway, maintain a steady speed in top gear. Keep tires at the proper inflation and wheels aligned. Don't pile luggage on a roof rack, which increases drag. Use a properly fitted gas cap. Keep your engine tuned.
Does speed make that much difference?
The Energy Department says gas mileage usually drops over 60 mph, and that each 5 mph over 60 is like paying an additional 20 cents per gallon of gas. Consumer Reports' Camry dropped from 40 mpg to 35 when its speed rose from 55 to 65. At 75 mph it lost another 5 mpg. One reason is that aerodynamic drag increases the faster you drive, requiring more fuel to power the car through the air.
Information from Consumer Reports, the (Ottawa, Ill.) Times and the Department of Energy was used in this report.