8 Driving Habits That Quietly Lower Gas Mileage According To The Department Of Energy, And Which Ones Cost The Most

Gas prices are out of your hands, but your right foot is not. The Department of Energy’s fuel economy site spells out which everyday habits quietly eat into your mileage, and a few of them cost far more than drivers realize.

A car dashboard with the speedometer needle near 100 while driving.

The figures below come from the driving habits page on fueleconomy.gov and its maintenance page, the government’s fuel economy resource run with the Department of Energy. The site says its estimates are based mainly on conventional gasoline vehicles and are less accurate for diesels, hybrids and vehicles with cylinder deactivation.

Here are 8 habits, ordered roughly from the biggest hit to the smallest.

1. Aggressive Driving, Which Costs 15% To 30% On The Highway

Speeding, rapid acceleration and hard braking lower mileage roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic, according to the site. It puts the equivalent savings from driving sensibly at $0.29 to $1.18 a gallon.

Driver feedback devices improve average fuel economy about 3%, and drivers who use them to save fuel can improve it about 10%. The site adds that sensible driving is also safer for you and others.

2. Driving Faster Than The Speed Limit

Gas mileage usually drops rapidly above 50 mph, the site says. Each 5 mph over 50 is like paying an extra $0.21 a gallon.

Slowing down 5 to 10 mph gives a 7% to 14% fuel economy benefit, or $0.21 to $0.41 a gallon in savings, assuming fuel at $2.94 a gallon.

3. Hauling Cargo On The Roof

A large, blunt rooftop cargo box reduces fuel economy about 2% to 8% in city driving, 6% to 17% on the highway and 10% to 25% at 65 to 75 mph. That is a wide range, and the worst numbers come at highway speed.

Rear-mounted cargo boxes or trays cost less, about 1% to 2% in the city and 1% to 5% on the highway. The site says to remove external cargo containers when not in use, and puts the equivalent savings from removing roof cargo at $0.06 to $0.50 a gallon.

4. Letting The Engine Idle

Idling uses about a quarter to a half gallon of fuel per hour, depending on engine size and air conditioning use, per fueleconomy.gov. Restarting uses about 10 seconds’ worth of fuel.

The site says any shutdown longer than a minute saves money, as long as you keep to about 10 engine starts a day on average, unless the vehicle has a start-stop system.

The site also puts idling at about $0.01 to $0.02 a minute with the air conditioning off and $0.02 to $0.03 a minute with it on. It adds that you should drive at least 5 miles between start cycles to fully recharge the battery, and limit electric accessory use during long shutdowns.

5. Carrying Extra Weight

Each extra 100 pounds reduces mpg about 1%, or about $0.03 a gallon, the site says. The effect is larger on smaller vehicles.

That golf bag, toolbox or load of old boxes in the trunk is costing you at every fill-up.

6. Skipping Cruise Control On The Highway

Cruise control helps maintain a constant speed and, in most cases, saves gas, according to fueleconomy.gov. The site gives no percentage for it, but steady speed avoids the speeding up and slowing down that drag mileage down.

That is one of the easiest habits to adopt. Pick a highway speed you can hold, set the control, and let the car do the work on flat, open stretches.

7. Underinflated Tires

The maintenance page says proper inflation improves gas mileage by 0.6% on average, and up to 3% in some cases. Each 1 psi drop in average pressure reduces mileage by roughly 0.2%.

Because tires lose pressure slowly, you may not feel the difference at the wheel. It shows up at the pump, a little at a time, which is why the DOE page lumps it in with maintenance.

Follow the pressure on the door-jamb sticker or in the owner’s manual, not the maximum printed on the tire.

8. Using The Wrong Motor Oil Grade

Using the manufacturer’s recommended grade improves mileage by 1% to 2%, equivalent to $0.04 to $0.08 a gallon, the site says. It advises looking for the “Energy Conserving” symbol on the API label.

The site also mentions that a check engine light deserves a mechanic’s inspection, though it gives no percentage for that one.

So which habits cost the most? Going by the site’s percentages, aggressive driving and a rooftop cargo box at highway speed are the two biggest hits, followed by speeding. Weight, tire pressure and oil grade are smaller, but they are also nearly free to fix.

Note that the dollar figures on the site assume different fuel prices on different pages, $2.94 a gallon on the speed section and $3.99 on the maintenance page, so compare the percentages rather than the dollars. If you drive a hybrid, diesel or plug-in, treat the numbers as rough guides.

A few minutes now pays back all year. Unload the trunk, take the cargo box off the roof when it is not in use, check your tire pressure at least monthly, and ease into the gas pedal. Your wallet will feel it long before your car does.

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