Miles per Gallon
Calculate MPG and optional fuel costs.
About the Miles per Gallon
Miles per gallon from the two numbers you actually have: distance travelled and fuel used.
How to use it
- Enter the miles driven.
- Enter the gallons used to cover them.
- Read your real-world MPG.
The formula
Fuel economy is one division. What it costs you is a second one, and that is the number worth acting on.
MPG = Miles driven ÷ Gallons used
Cost per mile = Fuel price ÷ MPG
Annual fuel cost = Annual miles ÷ MPG × Fuel price
To measure real MPG rather than what the dashboard claims, fill the tank completely, reset the trip meter, drive normally, then fill completely again. The gallons on the second receipt are exactly what the trip consumed, because you have returned the tank to the same level. One tank is a noisy sample; three consecutive tanks averaged is a genuinely reliable figure.
Worked examples
| MPG | Annual miles | Fuel price | Gallons a year | Annual cost |
|---|---|---|---|---|
| 18 | 12,000 | $3.50 | 666.7 | $2,333 |
| 25 | 12,000 | $3.50 | 480.0 | $1,680 |
| 28.3 | 12,000 | $3.50 | 424.0 | $1,484 |
| 35 | 12,000 | $3.50 | 342.9 | $1,200 |
The rows are evenly spaced in MPG and not in money, and that is the trap in fuel economy. Going from 18 to 25 MPG saves $653 a year; going from 28 to 35 saves $284, despite being the same seven-mile improvement. Because cost depends on gallons per mile rather than miles per gallon, upgrades at the thirsty end of the range are worth far more than the same gain at the efficient end.
Common mistakes
- Trusting the dashboard readout. On-board computers estimate, and they are frequently optimistic by a few percent. Measuring at the pump with the tank returned to the same level is the honest method.
- Measuring a single tank. One tank of motorway driving flatters the figure and one of short cold trips wrecks it. Average three consecutive tanks before believing the result.
- Not filling to the same point both times. The whole method depends on the tank being at an identical level at the start and end. Stopping at the first click each time is the practical way to make that repeatable.
- Comparing MPG figures across countries. A US gallon is 3.785 litres and an imperial gallon is 4.546. UK MPG figures are therefore about 20% higher for identical consumption, which makes cross-market comparisons meaningless unless you convert.
Terms explained
- MPG
- Miles travelled per gallon consumed. Higher is better, and the scale is misleading because it is a reciprocal.
- GPM
- Gallons per hundred miles — the inverse measure. It scales linearly with cost, which is why some regulators prefer it.
- Cost per mile
- Fuel price divided by MPG. The most directly useful number for budgeting or comparing two vehicles.
- Combined rating
- A weighted blend of official city and highway figures, usually about 55% city. Real driving rarely matches the mix.
- Imperial gallon
- 4.546 litres, against the US gallon's 3.785. The reason British MPG figures look better than American ones.
- MPGe
- Miles per gallon equivalent, used for electric vehicles by converting kilowatt-hours into the energy content of a gallon of petrol.
Common questions
- How do I measure my real MPG?
- Fill the tank completely, reset the trip meter, drive as you normally would, then fill completely again. Divide the trip miles by the gallons it took to refill. Average three tanks for a figure you can trust.
- Why is my MPG worse than the official rating?
- Official figures come from standardised tests. Cold weather, short trips, roof boxes, underinflated tyres, heavy loads and brisk acceleration all cost real fuel that the test cycle does not capture.
- Does driving style really make a difference?
- A large one. Hard acceleration and high motorway speeds are the two biggest levers — fuel use rises steeply above roughly 55 to 60 mph because air resistance grows with the square of speed.
- Is it worth upgrading to a more efficient car?
- Compare annual fuel costs, not MPG figures. The saving is largest when replacing a thirsty vehicle: 18 to 25 MPG saves more than 28 to 35, even though the MPG gain is identical.
- Does tyre pressure matter?
- Yes, though modestly — underinflated tyres typically cost a few percent in economy, plus faster wear and worse handling. It is the cheapest efficiency measure available.
- Why do city and highway figures differ so much?
- Stopping and starting wastes the energy used to accelerate, and idling consumes fuel while covering no distance. Steady cruising avoids both, which is why highway figures are usually higher for conventional cars — and often lower for hybrids, which recover braking energy in town.
- How do I convert to litres per 100 km?
- Divide 235.21 by your US MPG. So 28.3 MPG is about 8.3 litres per 100 km. The conversion constant differs for imperial gallons.
- Does premium fuel improve economy?
- Only in engines that require or can exploit a higher octane rating. In a car designed for regular fuel, premium generally buys nothing but a larger bill.