Gas Mileage Calculator
Calculate MPG and fuel efficiency for your vehicle.
Related calculators
About
Gas Mileage Calculator
The simplest way to work out gas mileage is to note the odometer reading, or reset the trip counter, when you fill the tank. At the next fill-up, take the distance covered between the two readings and divide it by the fuel you put in the second time.
That second point matters. You divide by the fuel added at the end of the interval, not the start, because that is the fuel that replaces what the trip consumed. Fill the tank both times and the amount you add the second time is what the distance actually cost you.
For the worked example above, an odometer moving from 12,000 to 12,360 miles on 15 gallons gives 360 ÷ 15 = 24 mpg, which is 9.8 L/100 km or 10.2 km/L. At $3 a gallon the fill costs $45, which works out at 12.5 cents a mile.
The three ways of expressing it
Miles per gallon and kilometres per litre are distance per unit of fuel, so bigger is better. Litres per 100 km is fuel per unit of distance, so smaller is better. They describe the same thing inverted, and the inversion has a consequence most people miss.
Because mpg is a reciprocal, equal steps in mpg are not equal savings in fuel. Over 10,000 miles a year:
| Improvement | Gallons before | Gallons after | Saved |
|---|---|---|---|
| 10 to 15 mpg | 1,000 | 667 | 333 |
| 15 to 20 mpg | 667 | 500 | 167 |
| 20 to 25 mpg | 500 | 400 | 100 |
| 30 to 35 mpg | 333 | 286 | 48 |
| 40 to 45 mpg | 250 | 222 | 28 |
Going from 10 to 15 mpg saves 333 gallons. Going from 30 to 35, the same five-mpg step, saves 48. Improving a thirsty vehicle is worth roughly seven times more fuel than the same nominal improvement on an efficient one, which is why replacing an old truck matters more than upgrading an already economical car.
This is exactly the trap that litres per 100 km avoids, and it is why most of the world states consumption that way. Fuel per distance is linear: halving the number halves the fuel.
US gallons and imperial gallons
A US gallon is 3.785411784 litres and an imperial gallon is 4.54609, about 20% larger. The same car therefore posts a higher figure in imperial mpg without being any more economical: 24 US mpg is 28.82 imperial mpg.
Comparing a British review with an American one without converting will make the British car look better every time. The calculator reports both, plus km/L and L/100 km, so the comparison is like for like.
Getting a reading you can trust
A single tank is a noisy measurement. Fill-ups stop at different points depending on the pump, the nozzle cut-off and how level the ground is, and a partial fill throws the whole figure off.
Fill the tank completely both times, use the same pump if you can, and take the average over three or four tanks rather than trusting one. A tank that includes an unusual amount of motorway driving, or a cold week, will not represent normal use.
Trip computers are convenient and frequently optimistic. They estimate flow rather than measuring what went into the tank, and errors of a few percent in the driver's favour are common. The odometer method above is slower and more honest.
How to improve gas mileage
Carpooling, public transport, walking and cycling are the best options for anyone looking for the best possible fuel efficiency. Where those are not viable, these are the levers that actually move the number.
- Drive a fuel-efficient vehicle. Four-wheel drive vehicles tend to be less efficient than front or rear wheel drive, since powering all four wheels makes the engine work harder to hold the same speed.
- Tires. Under-inflated tires can lower gas mileage by about 0.3% per PSI of drop, because energy is wasted deforming a soft tire instead of moving the car. A rigid tire gives better frictional interaction with the road and carries momentum forward. Check the driver's manual or the markings on the tire sidewall for the correct range, and keep the wheels aligned.
- Around 55 mph is the sweet spot for gas vehicles. Studies by the U.S. Department of Energy put the optimal speed for fuel efficiency at 55 mph, though it varies by vehicle. Cruise control helps on long, straight, rural roads by removing unnecessary acceleration and deceleration.
| Speed | Metric | Less efficient than 55 mph |
|---|---|---|
| 60 mph | 97 km/h | 3% |
| 65 mph | 105 km/h | 8% |
| 70 mph | 113 km/h | 17% |
| 75 mph | 121 km/h | 23% |
| 80 mph | 129 km/h | 28% |
Drag rises with the square of speed while the power needed to overcome it rises with the cube, which is why the penalty accelerates so sharply past 70.
- Don't be aggressive. Steadiness wins. Hard acceleration churns the engine and burns more fuel, and hard braking simply throws away the energy you just bought, while wearing out the brakes. Driving smoothly can improve gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic.
- Keep the vehicle in good shape. Using the recommended motor oil alone is worth 1% to 2%. If the manual specifies 5W-30, do not use 10W-30. Keep the engine tuned, make sure the oxygen sensor is working, fix a failed emissions test promptly, and check the air filter every few months, since debris drawn in from outside restricts airflow to the engine.
- Remove unnecessary weight. The heavier the object, the more energy is needed to move it. A smaller, lighter car is a good start, but anyone benefits from clearing out heavy items that are not in use. Leaving a trailer hitched for the daily commute is the same mistake in a larger form.
- Roll up the windows. Open windows add drag, and at highway speed drag is the dominant force working against the car. Highway driving is usually more efficient with the windows up, even running the air conditioning instead. Roof racks and bike racks add both weight and drag, and are worth removing when not in use.
- Air conditioning. Fuel economy suffers in hot climates because of the energy diverted to run the compressor; economy can drop by as much as 25%. Avoid running it harder than necessary, particularly while the car is idle, since AC systems are more efficient when the car is moving.
- Avoid unnecessary idling. An idling vehicle gets 0 mpg. Modern engines do not need lengthy warm-ups, and restarting uses less fuel than idling for more than about a minute.
These habits save money on fuel and on maintenance, and being conscious of fuel economy carries real environmental benefits alongside the financial ones.
Where the fuel actually goes
Only a small share of the energy in the tank reaches the wheels. In a conventional gasoline car it is roughly a fifth, and most of the rest leaves as heat through the exhaust and the radiator, because a petrol engine is a heat engine and heat engines are thermodynamically limited.
The remainder is divided between the drivetrain, accessories such as the alternator and the water pump, and idling, which converts fuel into nothing at all. Aerodynamic drag and rolling resistance then consume most of what does arrive at the wheels, in a ratio that shifts with speed: rolling resistance dominates around town, drag dominates on the motorway.
That split explains why the advice above works the way it does. Tire pressure matters most at low speeds, where rolling resistance is the larger loss. Windows, roof racks and speed itself matter most on the motorway, where drag is. And nothing at all helps while the engine is idling, since the denominator of the mileage calculation has stopped moving.
Common mistakes
The first is dividing by the fuel put in at the start of the interval rather than the end. The fuel that matters is the fuel that replaced what you burned.
The second is comparing mpg figures across measurement systems. An imperial mpg number is about 20% higher than the US figure for the same vehicle.
The third is judging efficiency on one tank. Use several, and fill fully every time.
The fourth is treating equal mpg gains as equal savings. Five mpg at the bottom of the range is worth many times five mpg at the top.
Common questions
Frequently asked questions
Note the odometer reading when you fill the tank, then note it again at the next fill-up and divide the distance covered by the fuel added the second time. Going from 12,000 to 12,360 miles on 15 gallons gives 360 divided by 15, or 24 mpg.
The second. That fuel is what replaced the amount the trip actually consumed. Filling the tank completely both times is what makes the arithmetic work.
Divide 235.214583 by the mpg figure. 24 mpg gives 9.8 L/100 km. To get kilometres per litre instead, multiply the mpg by 0.4251, which turns 24 mpg into 10.2 km/L.
Because it is linear. Fuel per distance halves when consumption halves, whereas mpg is a reciprocal, so equal steps do not represent equal savings. Going from 10 to 15 mpg saves 333 gallons over 10,000 miles; going from 30 to 35 saves only 48.
The imperial gallon is 4.54609 litres against 3.785411784 for the US gallon, about 20% larger, so the same car posts a higher imperial figure. 24 US mpg is 28.82 imperial mpg without a drop more efficiency.
Around 55 mph for most gas vehicles, according to U.S. Department of Energy studies. Relative to that, a vehicle is typically 3% less efficient at 60 mph, 8% at 65, 17% at 70, 23% at 75 and 28% at 80.
Under-inflation costs roughly 0.3% of gas mileage per PSI below the correct pressure, because energy goes into flexing a soft tire rather than moving the car. Use the figure in the manual or on the tire sidewall, and keep the wheels aligned.
On the highway, windows up with the AC on is usually more efficient, because drag dominates at speed. In slow traffic the opposite tends to hold, since AC can cost as much as 25% of fuel economy in hot conditions.
An idling vehicle achieves 0 mpg by definition, since it covers no distance. Restarting a modern engine uses less fuel than idling for more than about a minute, and long warm-ups are unnecessary.
Trip computers estimate fuel flow rather than measuring what went into the tank, and they tend to read a few percent optimistic. The odometer and pump method is slower and more honest. Average several tanks rather than trusting one.