Photo of a gas gauge to illustrate getting the best fuel economy.

Motorists face rising gasoline prices as the summer travel season gets underway, so fuel efficiency is as important as ever.

CR has gathered its best test-based tips and techniques for getting the most gas mileage from your car to help ease the pain at the pump.

The FAQ below addresses common questions we receive and dispels some fuel-economy myths.  

What's the Best Way to Cut Fuel Costs?

Slow down. In our tests we’ve found that driving faster on the highway can really take a bite out of a car’s fuel efficiency.

We measured gas mileage while driving at a steady 55, 65, and 75 mph in a Honda Accord, a Toyota RAV4, and three versions of a Ford Fusion, including a hybrid. The drop in fuel economy while speeding up from 55 to 65 was 4 to 8 mpg. Increasing the speed to 75 cut fuel efficiency by an additional 5 to 7 mpg.

More on Fuel Economy

Overall, speeding up from 55 mph to 75 is like moving from a compact car to a large SUV. Beyond fuel concerns, speeding is, of course, a safety risk as well.

Avoid hard acceleration and braking whenever possible. In our tests, frequent bursts of acceleration and braking reduced an older Toyota Camry’s mileage by 2 to 3 mpg. Once up to speed on the highway, maintain a steady pace.

Smooth acceleration and braking also extend the life of the engine, transmission, brakes, and tires.

What's the Impact of Carrying Stuff on the Roof?

Securing things on the roof increases aerodynamic drag, making the engine work harder and hurting fuel economy.

When we tested a  2013 Honda  Accord at a steady 65 mph, it got 42 mpg with nothing on the roof. Adding an empty bike rack dropped the mileage by 5 mpg. 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. Going across town, this may not be a concern. But on a multistate family road trip this summer, a carrier or rack on the roof would have a real impact on fuel costs. And it could create some annoying wind noise. 

Use AC or Open Windows?

The harder the AC system has to work, the worse the impact on fuel economy. When we measured the fuel-economy difference in a  2008 Ford Focus, Honda  Accord, and Subaru Forester, we found that fuel use with the  AC 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 AC on.

In general, expect a drop from 1 to 4 mpg with the air conditioning running. The effect of opening the windows at 65 mph was not measurable.

How Far Can You Go When My Low-Fuel Warning Light Comes On?

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 to 50 miles or so at a moderate speed.

To maximize those last couple of gallons, we suggest slowing down and maintaining a steady pace. Because you never know what challenges life will serve up, including traffic, don’t rely on the light as your cue to fill up. 

Can Running on Empty Hurt Your Engine?

Some people think that running on empty can bring debris into the engine from the bottom of the fuel tank, but it’s not really a big concern.

The fuel pump always pulls gas in from the bottom of the tank, even when it’s full. If there were a debris problem, it would surface 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. If there is junk in the gas tank, though, you may have to change the filters more frequently. 

Does a Dirty Air Filter Hurt Gas Mileage?

Changing the air filter probably won’t improve fuel economy with modern cars. 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 maintain the right air-fuel ratio. We expect similar results after any air-filter change.

Should You Warm Up the Car Before Driving?

Nope. That adage held in the days of carburetors, but it isn’t the case with modern fuel-injected, electronically controlled drivetrains.

Engines are most efficient when they’re at regular operating temperature, and the fastest way to reach that point is to drive right after starting the car. 

Should You Refuel When the Air Is Cool?

Any extra gas gained by refueling when temperatures are cooler will be negligible and not make a practical difference. A common myth is to buy gasoline in the morning, when the air is cool, rather than in the heat of the day.

The theory is that the cooler gasoline will be denser, providing more go for the money. But most stations store the gasoline underground, so its temperature changes very little, if at all, during a 24-hour stretch.

Do 'No Name' Gas Stations Offer Lower-Quality Fuel?

Independent stations usually buy their fuel from larger, name-brand oil companies, so it’s not much different from what is offered for a higher price down the road. Off-brand gasoline is sometimes formulated without additives designed to clean the engine, but your car should run fine on that gas.

Better-quality gas is considered “top tier,” and it is available from many brands, including some less popular ones.

Should You Treat Your Car to Premium Gas?

The vast majority of cars are designed to run fine on regular. When it comes to regular, midgrade, and premium gasoline, oil corporations have worked overtime to drill the “good, better, best” concept into our collective driver psyche.

Premium gas has a higher octane rating, usually 91 or above, making it more resistant to pre­ignition, a condition in which fuel burns uncontrollably in the engine. Higher-performing engines are the most susceptible to preignition because they tend to run hotter, which is why premium is often recommended or required for sports and luxury vehicles.

Premium also helps maximize power in high-performance engines. Without premium fuel, these engines might not deliver full power when accelerating or climbing hills. Most drivers will probably never notice the difference, especially in routine everyday driving.

Our advice: The best gas for your car depends on the vehicle. If the owner’s manual or the sticker on the fuel-filler door says that premium gas is recommended or uses similar wording, you can probably use regular. If it says premium is required, play it safe by using the correct octane. 

Learn more in "Why You Might Not Actually Need Premium Gas."

Should You Switch to Low-Rolling Resistance Tires?

The lower the tire’s rolling resistance, or road drag, the better the fuel economy will be. However, maintaining the proper tire pressure will optimize the rolling resistance and real-world performance for any tires—even the tires currently on a vehicle.

Some tires reduce resistance at the expense of wet-braking performance and tread life—a poor trade-off.

It’s better to look first for a tire that provides good all-around performance in important safety areas such as braking, handling, and hydroplaning resistance. Then use its rolling resistance level as the tiebreaker.

Don’t replace your tires too early in an effort to chase the pennies that low-rolling-resistance tires may save in fuel. (See our complete tire buying guide and ratings.)

What Role Does Tire Pressure Play?

A midsized sedan lost 1.3 mpg on the highway when the tires were under-inflated by 10 psi. That's a sizeable drop in pressure for a modest drop in mpg.

More important, though, under-inflated tires compromise a car's handling and braking. Plus, under-inflated tires wear faster and run much hotter, which can lead to tire failure.

For safety's sake, check tire pressure at least once a month. The owner's manual explains how to do it.