MPG Calculator
25 miles per gallon.
| Unit | Value | Used in |
|---|---|---|
| MPG (US) | 25 | United States |
| MPG (imperial) | 30.02 | UK — about 20% higher for the same car |
| L/100km | 9.41 | Most of the world — lower is better |
| km/L | 10.63 | Japan, India, parts of Latin America |
| Cost per mile at $3.50/gal | $0.14 | — |
| Cost per year 12000 miles | $1,680.00 | — |
To measure real economy, fill the tank completely, reset the trip meter, drive normally, then fill completely again — the gallons it takes to refill is what you actually used. One tankful is noisy; average three or four for a figure worth trusting. L/100km runs in the opposite direction to MPG: lower is better, because it measures fuel consumed rather than distance travelled. That inversion also means MPG improvements are not linear in savings — going from 15 to 20 MPG saves far more fuel over the same distance than going from 40 to 50, which is why replacing the thirstiest vehicle in a household matters more than upgrading the efficient one. Imperial MPG reads about 20% higher than US MPG for the identical car, purely because an imperial gallon is larger; comparing a UK figure to a US one without converting flatters the UK car.
Calculate your real fuel economy from miles and gallons, and convert between MPG, L/100km, km/L, and imperial MPG.
How to use this calculator
- Choose Measure my MPG to work it out from a tankful, or Convert units to translate a figure you already have.
- To measure: fill the tank completely, reset the trip meter, drive normally, then fill completely again. Enter the trip miles and the gallons it took to refill.
- Add a fuel price and annual mileage for cost per mile and per year.
The formula
MPG = Miles ÷ Gallons · L/100km = 235.21 ÷ MPG · Imperial MPG = US MPG × 1.201
Worked example — 300 miles on 12 gallons
- MPG (US): 300 ÷ 12 = 25.0
- L/100km: 235.21 ÷ 25 = 9.41
- km/L: 10.63
- MPG (imperial): 25 × 1.201 = 30.0
- At $3.50/gal: 14¢ per mile, or $1,680 a year at 12,000 miles
The unit that runs backwards
MPG and L/100km measure the same thing in opposite directions. MPG asks "how far on a fixed amount of fuel?" — higher is better. L/100km asks "how much fuel for a fixed distance?" — lower is better. Most of the world uses the second because it answers the more useful question, and because it behaves linearly in a way MPG does not.
That non-linearity has a real consequence. Because MPG is an inverse measure, equal-sized improvements do not save equal amounts of fuel:
| Improvement | Gallons for 12,000 miles | Gallons saved |
|---|---|---|
| 15 → 20 MPG | 800 → 600 | 200 |
| 20 → 25 MPG | 600 → 480 | 120 |
| 30 → 35 MPG | 400 → 343 | 57 |
| 40 → 50 MPG | 300 → 240 | 60 |
A 5 MPG improvement at the bottom of the range saves more than three times as much fuel as the same improvement at the top — and a 10 MPG jump from 40 to 50 still saves less than the 5 MPG jump from 15 to 20. If a household has a thirsty vehicle and an efficient one, replacing the thirsty one is where the savings actually are.
Why your real MPG is below the sticker
EPA ratings come from standardised test cycles run under controlled conditions. Useful for comparing one car to another; poor at predicting your fuel bill. The gap usually comes from:
- Speed. Aerodynamic drag rises with the square of speed. Economy typically falls sharply above about 65 mph.
- Short trips. A cold engine runs rich and inefficiently. Several short journeys use far more fuel than one long one covering the same distance.
- Cold weather. Denser air, thicker fluids, longer warm-up. Winter economy is routinely 10–20% below summer.
- Roof racks and boxes, which can cost 10–25% at highway speed even when empty.
- Tyre pressure. Under-inflation raises rolling resistance measurably.
- Driving style. Hard acceleration followed by hard braking discards energy you paid for.
Measuring it properly
The trip computer in the car is convenient but frequently optimistic — often by several percent. To get a real figure, use the tank-to-tank method: fill completely, reset the trip meter, drive, fill completely again at the same pump if you can, and divide.
Average at least three or four tanks. A single tankful is noisy: how full the pump cut off, the mix of city and highway, the weather, and even the elevation difference between two stations all move the number enough to mislead.
How we calculate this
MPG is miles divided by gallons. Conversions use exact definitions: a US gallon is 3.785411784 litres, an imperial gallon 4.54609 litres, and a mile 1.609344 kilometres. L/100km is therefore (100 × 3.785411784) ÷ (MPG × 1.609344), which is where the familiar 235.21 constant comes from. Because the relationship is inverse, the same constant converts in both directions. Imperial MPG is US MPG × 1.20095, reflecting nothing but the larger gallon.
Sources
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Frequently asked questions
How do I calculate MPG?
Divide the miles driven by the gallons used. Fill the tank completely, reset the trip meter, drive normally, then fill completely again — the gallons it takes to refill is exactly what you used. 300 miles on 12 gallons is 25 MPG.
How do I convert MPG to L/100km?
Divide 235.21 by the MPG figure. 25 MPG is 235.21 ÷ 25 = 9.41 L/100km. The relationship is inverse, so the conversion works in both directions with the same constant — 9.41 L/100km back to MPG is 235.21 ÷ 9.41 = 25.
Why is UK MPG higher than US MPG for the same car?
Because an imperial gallon is 4.546 litres and a US gallon is only 3.785. The same car travels about 20% further on an imperial gallon simply because the gallon is bigger. A car rated 30 MPG in the US is about 36 MPG in the UK, with no difference in efficiency whatsoever.
Is a lower L/100km better or worse?
Lower is better. L/100km measures fuel consumed over a fixed distance, so less is better — it runs in the opposite direction to MPG, which measures distance covered per fixed fuel. This trips people up constantly when comparing cars across markets.
Why doesn't my real MPG match the EPA rating?
EPA figures come from standardised test cycles, and real driving rarely resembles them. Short trips where the engine never warms up, cold weather, highway speeds above about 65 mph, roof racks, low tyre pressure, heavy loads, and aggressive acceleration all cut economy — often by 15–25% combined. The rating is useful for comparing cars, not for predicting your fuel bill.
Does improving MPG save the same amount at every level?
No, and this is the most counterintuitive thing about fuel economy. Going from 15 to 20 MPG saves far more fuel over the same distance than going from 40 to 50, because MPG is an inverse measure. Over 12,000 miles, 15 to 20 saves 200 gallons; 40 to 50 saves only 60. Upgrading the thirstiest vehicle in a household matters far more than improving the efficient one.
How can I improve my fuel economy?
Slow down — economy falls sharply above about 65 mph. Keep tyres at the correct pressure, which is worth a few percent on its own. Remove roof racks and boxes when not in use, since aerodynamic drag dominates at highway speeds. Combine short trips so the engine warms up once rather than repeatedly. Accelerate gently and anticipate rather than braking hard.
How many tankfuls should I average?
At least three or four. A single tank is noisy — how full the pump cut off, the mix of driving, the weather, and even the road grade between two fill-ups all move the figure. Averaging several tanks over a few weeks gives a number that reflects how you actually drive.