Auto & travel
Gas mileage calculator (MPG)
Fill the tank, drive, fill it again. Two odometer readings and the fuel you put in give you your true economy, which is the only figure that reflects how you actually drive.
In the same units as the result. Leave at zero to skip.
How to measure it properly
The tank-to-tank method is simple and, done consistently, accurate to within a couple of percent.
- Fill the tank completely and note the odometer.
- Drive normally. A few hundred miles gives a better average than one short trip.
- Fill completely again, at the same pump if you can, and note both the odometer and the litres or gallons added.
- Divide the distance by the fuel added.
The critical detail is filling to the same point both times. Pumps click off at different levels depending on flow rate and nozzle position, so “first click” is the usual convention. Topping off past the click on one fill but not the other is the main source of error, and it can easily distort a single tank by 5–10%.
Trip computers are not measuring this
Dashboard economy readouts estimate fuel flow from injector duty cycles rather than measuring what went into the tank. They are reasonable but not neutral, and independent testing has repeatedly found them optimistic by a few percent — often around 2–5%, occasionally more. If your calculated figure comes in below the dashboard number, the dashboard is the more likely culprit.
Averaging across tanks
A single tank tells you about a single tank. One long motorway run flatters the number; one week of school runs in January destroys it. Fuel economy varies enough between tanks that a single measurement should not be taken as your car’s figure.
The better approach is a running log: record odometer and litres at every fill for a few months, then divide total distance by total fuel. That also makes a developing mechanical problem visible, since a gradual decline in economy over several tanks is often the first sign of a failing sensor, dragging brake or clogged filter.
What changes the number
| Factor | Typical effect |
|---|---|
| 80 mph instead of 65 | 15–25% worse |
| Short trips from cold | Up to 30% worse |
| Roof box fitted | 10–25% worse at speed |
| Tyres significantly under-inflated | 2–3% worse |
| Air conditioning in heavy use | 3–10% worse |
| Winter fuel blend and cold air | A few percent worse |
Most of these are additive, which is why a cold February of short urban trips with a roof box still fitted can produce a figure 40% below the summer motorway number for the same car.
Common questions
How do I calculate MPG?
Divide the miles driven by the gallons of fuel added to refill the tank. If you covered 333 miles and it took 10.4 gallons, that is 32.0 MPG. Fill to the same point both times, since inconsistent filling is the main source of error.
Is my trip computer accurate?
Close, but usually a little optimistic. It infers fuel use from injector timing rather than measuring volume, and independent testing generally finds readouts overstate economy by a few percent. Calculating from pump receipts over several tanks gives a truer figure.
What is good fuel economy?
It depends entirely on the vehicle class. A modern small petrol hatchback typically returns 40–50 US MPG; a mid-size sedan 30–35; a full-size pickup or large SUV 18–22. Hybrids commonly exceed 50. Comparing against similar vehicles is more informative than any absolute threshold.
Why does my economy drop in winter?
Several effects stack up. A cold engine runs rich until it warms, so short trips never reach efficient operating temperature. Cold air is denser and increases drag, winter fuel blends carry slightly less energy, tyre pressures fall in the cold, and heating and defrosting draw power. A 10–20% winter drop is normal, and worse for short journeys.