There is no shortage of fuel-saving advice, and most of it is repeated without anyone ever putting a number next to it. So we went the other way round: start from the figures the US Department of Energy publishes on fueleconomy.gov, rank the eight tips by what they actually measure, and be honest about the one that turns out to do very little on a modern car.
At a glance
- Driving style is worth more than every other tip on this page combined.
- Every 5 mph above 50 mph costs the equivalent of about $0.29 per gallon.
- Under-inflated tyres cost roughly 0.2% of your mileage per psi lost, averaged across all four.
- Replacing a clogged air filter no longer improves fuel economy on a modern fuel-injected engine.
The three habits that move the needle
Fuel economy is decided mostly behind the wheel, not under the bonnet. These are the three with the largest published effects.
1. Stop accelerating and braking hard
Rapid acceleration and heavy braking are, by a wide margin, the most expensive habit in the car. According to the Department of Energy’s figures on fueleconomy.gov, aggressive driving lowers gas mileage by roughly 15% to 30% at highway speeds and by 10% to 40% in stop-and-go traffic. Nothing else here comes close.
The practical version is unglamorous. Look further ahead, lift off earlier, and let the car roll toward a red light instead of sprinting to it and standing on the brakes. Every bit of speed you scrub off with the brakes is fuel you already paid for and converted into heat.
2. Ease off above 50 mph
Aerodynamic drag climbs steeply with speed, so the last few miles per hour are always the most expensive ones. The Department of Energy estimates that each 5 mph driven above 50 mph costs roughly the equivalent of $0.29 per gallon at recent fuel prices. On a long motorway run, dropping from 80 to 70 mph is one of the few changes that shows up on the trip computer within minutes.
3. Do not idle to warm up
Idling produces zero miles per gallon by definition, and the Department of Energy puts consumption at a quarter to half a gallon of fuel per hour depending on engine size and air conditioning use. Modern engines warm up faster under gentle load than sitting still, so the useful rule is simple: if you expect to be stationary for more than a minute and it is safe to do so, switch off.
The eight tips, ranked by measured effect
| Tip | Published effect | Effort |
|---|---|---|
| 1. Smooth throttle and braking | 15% to 30% highway, 10% to 40% in traffic | Free, but takes a few weeks to become a habit |
| 2. Hold a lower cruising speed | About $0.29/gallon equivalent per 5 mph over 50 mph | Free, costs you time instead |
| 3. Remove an unused roof box | 6% to 17% on the highway, up to 25% at 65-75 mph | Ten minutes, once |
| 4. Avoid needless idling | 0.25 to 0.5 gallon per hour stationary | Free |
| 5. Keep tyres correctly inflated | About 0.2% lost per psi below spec; 0.6% gained on average | Five minutes a month |
| 6. Use the recommended oil grade | 1% to 2% | Costs nothing extra at service time |
| 7. Drop dead weight from the boot | About 1% per extra 100 lb (roughly 45 kg) | One afternoon of honesty |
| 8. Combine short trips into one | No single published figure; cold engines are measurably thirstier | Planning only |
Figures above are from the US Department of Energy and EPA, published on fueleconomy.gov. The dollar equivalent for speed reflects the fuel price used by that source at the time of publication, so treat it as a ratio rather than as a local price.
Tyres, weight and roof racks: the physical losses
These three are the ones you can see and touch, which is probably why they get repeated so often. They are worth doing, but the numbers put them in perspective.
Tyre pressure loses you about 0.2% of your mileage for every 1 psi drop in the average pressure across all four tyres, and properly inflated tyres gain around 0.6% on average, according to fueleconomy.gov. That is small per psi and meaningful when a tyre has quietly lost six or seven of them over a winter, which is common. Check them cold, once a month, against the placard in the driver’s door shut.
Weight costs roughly 1% of mileage per extra 100 pounds, about 45 kg. In a small car that is not nothing, and most of us are carrying a boot full of things that have not been used since spring.
Roof boxes and racks are the surprise on this list. A roof cargo box costs 2% to 8% in town, 6% to 17% on the highway, and 10% to 25% at interstate speeds of 65 to 75 mph. A rear-mounted carrier costs far less, 1% or 2% in town and 1% to 5% on the highway. If the box lives on the roof year round, taking it off is one of the largest single gains available to most drivers.
The tip we had to drop: the air filter
This is the one that surprised us. Replacing a clogged air filter is repeated in almost every fuel-saving list, and on a modern car it is no longer true. The Department of Energy states plainly that on fuel-injected, computer-controlled petrol engines a new air filter does not improve fuel economy, although it can improve acceleration. The old advice comes from carburettored engines, where a blocked filter genuinely richened the mixture.
Change the filter anyway, on schedule, because it protects the engine and the intake. Just do not expect it to show up at the pump. The oil grade in the same service, by contrast, is worth that measurable 1% to 2%.
What actually happened when we tried this
The honest lesson from applying all eight is that the free ones are the hard ones. Buying a better oil grade takes a single conversation at the service desk. Rebuilding your throttle habits takes weeks, and the trip computer is the only thing that keeps you honest, because the improvement disappears the moment you stop paying attention on a bad Monday morning.
The other thing worth saying: none of these figures stack neatly. Losing 30% to aggressive driving and gaining 0.6% from correct tyre pressures are not opposites of the same kind. Fix the driving first, then the rest is genuinely worth the twenty minutes it costs.
Common questions about fuel economy
Is air conditioning or open windows more efficient?
It depends on speed. At low urban speeds, open windows cost little and the compressor load is the bigger penalty. At motorway speeds, the aerodynamic drag from open windows usually outweighs the air conditioning, so closed windows and the compressor are the better choice.
Does cruise control save fuel?
On flat roads, yes, because it holds a steady speed better than most drivers do. On hilly routes it can be worse than a thoughtful driver, since it will hold the set speed up a gradient by demanding more throttle than you would.
Do fuel additives and economy devices work?
Treat unverified claims with suspicion. If a device promised double-digit gains and delivered them, it would be fitted at the factory, because manufacturers are under regulatory pressure to reduce consumption. Stick to the measured items above.
How much does a cold engine really cost?
Short trips from cold are the least efficient driving most people do, because the engine spends the whole journey warming up and never reaches its efficient operating temperature. Grouping errands into one longer run is the practical fix.
Heading somewhere further than the supermarket?
Fuel habits are half the story on a long drive. The other half is making sure the car is ready before you leave.
Last updated: 4 August 2026. All percentage figures cited above come from the US Department of Energy and the Environmental Protection Agency, published on fueleconomy.gov (driving habits and vehicle maintenance pages). Results vary with vehicle, load, route and climate, so treat them as orders of magnitude rather than guarantees.