The Fuel Saving Habits That Actually Work

Fuel-saving advice is mostly folklore, and the folklore is expensive in a different way: it makes people do fiddly things with little effect while ignoring the two that matter. The EPA and the Department of Energy publish measured figures for each behaviour on fueleconomy.gov. Here they are, largest first.

Key figures

• 15 to 30 percent Highway fuel economy lost to aggressive driving
• $0.29 Per-gallon penalty for each 5 mph above 50 mph

1. How you drive: up to 40 percent

Speeding, rapid acceleration and hard braking lower fuel economy by roughly 15 to 30 percent at highway speeds and 10 to 40 percent in stop-and-go traffic. Nothing else on this list is in that range.

The practical version is not driving slowly. It is driving smoothly: looking further ahead, lifting off earlier, letting the car coast to the red light instead of accelerating towards it and braking. The fuel you spend accelerating to a stop sign is all converted to heat in the brakes.

2. How fast: about 29 cents a gallon per 5 mph

Fuel economy usually peaks somewhere between 50 and 60 mph and falls away above that, because air resistance rises with the square of speed. The EPA’s way of expressing it is the most useful: each 5 mph you drive over 50 mph is like paying an additional $0.29 per gallon, with the overall benefit of slowing down put at 7 to 14 percent.

At 75 mph against 65 mph on a 300-mile trip, you save around 37 minutes of driving. Price that against the fuel and decide.

• Aggressive driving, stop-and-goup to 40%
• Aggressive driving, highwayup to 30%
• Roof box at 65 to 75 mph10 to 25%
• Roof box, highway6 to 17%
• Rear-mounted carrier, highway1 to 5%
• Correct tyre pressure0.6% typical, up to 3%

Ranges published on fueleconomy.gov by the US Department of Energy and the EPA. Figures are the reduction in fuel economy; where a range is given, the upper end is shown.

3. What is on the roof: up to 25 percent

A large roof-top cargo box reduces fuel economy by 2 to 8 percent in city driving, 6 to 17 percent on the highway, and 10 to 25 percent at interstate speeds of 65 to 75 mph. A rear-mounted cargo box or tray costs far less: 1 or 2 percent in the city and 1 to 5 percent on the highway.

The lesson is not to avoid carrying things. It is to take the empty box off the roof when the holiday is over, and to prefer a rear-mounted carrier if you have the choice.

4. Idling: a quarter to half a gallon an hour

Idling burns a quarter to a half gallon of fuel per hour depending on engine size and air conditioning use, and delivers nothing. The EPA prices it at 2 to 3 cents a minute with the air conditioning off, and 3 to 5 cents with it on.

Modern engines do not need to be warmed up at idle. The fastest way to bring an engine to operating temperature is to drive it gently. Where a stop will be more than a minute or so and it is safe to do it, switching off is worth more than most of what people do to save fuel.

5. Weight: about 1 percent per 100 pounds

An extra 100 pounds could reduce fuel economy by about 1 percent, and it matters proportionally more on small cars than large ones. This is the item most over-sold in advice columns: clearing the boot is worth doing, but it is not going to transform anything unless what is in the boot is genuinely heavy.

6. Maintenance: a few percent, cheaply

• Tyre pressure. Keeping tyres inflated to the pressure on the door jamb placard improves economy by about 0.6 percent on average, up to 3 percent, and fuel economy falls about 0.2 percent for every 1 psi drop across all four tyres.
• Oil grade. Using the manufacturer’s recommended grade improves economy by 1 to 2 percent. Using 10W-30 where 5W-30 is specified costs 1 to 2 percent; 5W-30 where 5W-20 is specified costs 1 to 1.5 percent.
• Air filter. On a modern fuel-injected engine, replacing a clogged filter improves acceleration but not fuel economy. Replace it when dirty, for the engine, not for the mpg.

What does not work

Coasting in neutral, which is illegal in some states and saves nothing on a fuel-injected engine that already cuts fuel on overrun. Aftermarket fuel-saving devices and additives, which have been the subject of FTC enforcement for decades. Filling up at dawn because the fuel is cold, which does not measurably change what you get. And overinflating tyres beyond the placard pressure, which trades a fraction of a percent for grip and uneven wear.

The cost-per-mile guide adds insurance, maintenance and ownership expenses to fuel spending. The hybrid premium comparison applies fuel savings to a separate purchase-price decision.

Questions people ask

How much does driving fast really cost?

The EPA's figure is that each 5 mph over 50 mph is like paying about 29 cents more per gallon, and that slowing down is typically worth 7 to 14 percent in fuel economy. Aggressive acceleration and braking cost more still: 15 to 30 percent at highway speeds.

Does a roof box use more fuel?

Substantially. The EPA and DOE put the reduction at 2 to 8 percent in city driving, 6 to 17 percent on the highway and 10 to 25 percent at 65 to 75 mph. A rear-mounted carrier costs only 1 to 5 percent on the highway.

Should I turn the engine off when waiting?

For anything longer than about a minute, where it is safe to do so. Idling burns a quarter to half a gallon an hour, which the EPA prices at 2 to 5 cents a minute depending on air conditioning use.

Does tyre pressure really save fuel?

A little: about 0.6 percent on average and up to 3 percent, with roughly 0.2 percent lost per psi of under-inflation. The bigger reasons to keep pressures correct are even wear and heat.

Where these figures come from

1. Driving More Efficiently US Department of Energy and EPA, fueleconomy.gov · fueleconomy.gov
2. Keeping Your Vehicle in Shape US Department of Energy and EPA, fueleconomy.gov · fueleconomy.gov
3. Weekly Retail Gasoline and Diesel Prices US Energy Information Administration · eia.gov

Figures last checked September 2026.