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Gas-saving tips & myths

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As the summer travel season rolls in, prices at the gas pump are usually going in the wrong direction for our wallets. That’s when drivers become more concerned about how to squeeze the most miles from their fuel dollars and keep their cars running their best.

To help you stay in the know, here are some common questions that our auto experts often get asked about gas mileage and related topics:

Slow down. In our tests, we’ve found that driving faster on the highway can really take a bite out of your car’s fuel efficiency. We measured gas mileage while driving at a steady 55, 65, and 75 mph in a Honda Accord, Toyota RAV4, and three versions of a Ford Fusion, including a hybrid. The drop in fuel economy while going from 55 to 65 rangedfrom 4 to 8 mpg. Upping the speed from 65 to 75 cut it 5 to 7 mpg more. Overall, speeding up from 55 mph to 75 is like moving from a compact car to a large SUV.

Carrying things on the roof increases aerodynamic drag, which hurts fuel economy. When we tested a  2013 Honda  Accord at a steady 65 mph, it got 42 mpg with nothing on the roof. Adding even an empty bike rack dropped the mileage by 5 mpg, to 37. A wind deflector reduced the wind noise but cut gas mileage to 35 mpg. And with two bikes on the rack, gas mileage dropped to 27 mpg, a whopping 15-mpg difference overall. Similarly, when we tested a  2008 Camry with a large car-top carrier, fuel economy dropped by 5 mpg.

It depends on how hard the air-­conditioning system has to work. When we measured the fuel-economy difference in a  2008 Ford Focus, Honda  Accord, and Subaru Forester, we found that fuel use with the  A/C running went up with higher outside temperatures.  At 55° F, there were negligible differences. But when we measured again on days when the temperature was in the low 70s and high 80s, we got fewer miles per gallon with the  A/C on. In general, expect 1 to 4 mpg less with air conditioning.

Visit our complete guide to fuel economy for more tips.  

There is no set rule, but most cars have a reserve of between 1 and 2 gallons of gas when the light goes on, or enough to travel about 40 miles or so at a moderate speed. To maximize those last couple of gallons, we suggest slowing down and maintaining a steady pace.

With modern cars, changing your air filter probably won’t improve your fuel economy. When we tested a car to see whether a dirty air filter hurt its gas mileage because of reduced air intake, we found that the car’s acceleration was hurt but not its fuel economy. The engine’s computer automatically compensated for the restricted airflow by reducing fuel use to maintainthe right air /fuel ratio. We expect similar results from any air-filter change.

Some people think that can draw in debris from the bottom of the fuel tank, but it’s not really a big concern. That’s because the fuel pump always pulls in gas from the bottom of the tank, even when it’s full. So if there is a debris problem, you’ll probably know about it long before the fuel level gets low. These days, there’s usually a fuel filter in the gas tank as well as one nearer the engine, so debris is unlikely to get through to your engine. If your tank contains junk, though, you might have to change the filters more frequently.

This article also apeared in the June 2014 issue of Consumer Reports magazine.


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