How the Mileage Calculator Works
Fuel mileage tells you how far your vehicle goes on a certain amount of fuel. It shows how well your car uses fuel, and it helps you to plan the cost of a trip, to compare cars, and to notice when something is wrong. The Mileage Calculator finds your fuel mileage from a real tank of fuel, and shows it in every common unit.
What you enter
- The distance, either as the odometer reading at the start and at the end of a tank of fuel, or directly as the distance you drove. It can be in miles or in kilometers.
- The fuel you used, in US gallons, UK (imperial) gallons or liters.
- The price of the fuel (optional), for one gallon or one liter, to work out the cost.
How the results are worked out
distance = ending odometer - starting odometer miles per gallon = miles / gallons kilometers per liter = kilometers / liters liters per 100 km = liters / kilometers x 100
Before the units are compared, everything is changed to kilometers and liters. One mile is 1.609344 kilometers. One US gallon is 3.785 liters, and one UK gallon is 4.546 liters.
A worked example
Your odometer shows 10,000 miles at the start and 10,450 miles at the end, and you put 15 US gallons of fuel in the tank.
| Result | Value |
|---|---|
| Distance (10,450 - 10,000) | 450 miles (724.2 km) |
| Fuel | 15 US gallons (56.78 L) |
| Miles per US gallon (450 / 15) | 30 mpg |
| Miles per UK gallon | 36.03 mpg |
| Kilometers per liter | 12.75 km/L |
| Liters per 100 km | 7.84 L/100 km |
With fuel at $3.50 a gallon, the trip cost $52.50. That is about $0.117 for each mile, or $11.67 for every 100 miles.
Metric example
You drove 600 km and put in 40 liters. That gives 600 / 40 = 15 km/L. In liters per 100 km it is 40 / 600 x 100 = 6.67. In miles per US gallon it is 35.28.
The units are not the same
Miles per gallon is a mileage: a higher number is better. Liters per 100 km is a consumption: a lower number is better. They are linked by a simple rule:
L/100 km = 235.215 / mpg (US) mpg (US) = 235.215 / (L/100 km) mpg (UK) = mpg (US) x 1.2009
| US mpg | UK mpg | L/100 km |
|---|---|---|
| 15 | 18.0 | 15.68 |
| 20 | 24.0 | 11.76 |
| 25 | 30.0 | 9.41 |
| 30 | 36.0 | 7.84 |
| 35 | 42.0 | 6.72 |
| 40 | 48.0 | 5.88 |
| 50 | 60.1 | 4.70 |
The UK gallon is about 20% bigger than the US gallon, so the same car has a bigger number in UK mpg. Always check which gallon a figure uses. A car that does 30 mpg in the United States does 36 mpg by the UK measure.
What the mileage costs
The cost of driving falls quickly when the mileage improves. At $3.50 a gallon, for every 100 miles:
| Mileage | Cost per 100 miles | Fuel for 12,000 miles a year | Cost for a year |
|---|---|---|---|
| 15 mpg | $23.33 | 800 gallons | $2,800 |
| 20 mpg | $17.50 | 600 gallons | $2,100 |
| 25 mpg | $14.00 | 480 gallons | $1,680 |
| 30 mpg | $11.67 | 400 gallons | $1,400 |
| 40 mpg | $8.75 | 300 gallons | $1,050 |
| 50 mpg | $7.00 | 240 gallons | $840 |
15 mpg ████████████████████████████████████████ $23.33 20 mpg ██████████████████████████████ $17.50 25 mpg ████████████████████████ $14.00 30 mpg ████████████████████ $11.67 40 mpg ███████████████ $8.75 50 mpg ████████████ $7.00
Moving from 25 to 30 mpg saves $280 a year for 12,000 miles. A small gain matters more at a low mileage than at a high one, because the fuel used is the inverse of the mileage.
The scale
The calculator draws your mileage on a scale from 0 to 60 miles per US gallon. It is a rough guide for passenger cars, and it is not a rating. Small cars and hybrid cars can be much higher, and large trucks and vans can be lower. Each block below is 1.5 mpg:
████████████████████████████████████████ 0 mpg 20 30 60
- Red, under 20 mpg: thirsty. Common for large trucks, SUVs and older cars.
- Amber, 20 to 30 mpg: average for many cars.
- Green, over 30 mpg: efficient.
How to get an accurate figure
- Fill the tank to the top, and write down the odometer. This is the start.
- Drive as usual, then fill the tank to the top again, at the same pump if you can, and the same way. Write down the odometer and the fuel that you added. This is the fuel that you used.
- Repeat for several tanks, and use the average. One tank can be unusual, because the fill level is not exact, and the mixture of city and highway driving changes.
The fuel figure from the trip computer of your car can be a few percent different from the real one, so a calculation from the pump is more reliable.
How to improve your mileage
- Drive at a steady speed. Fast acceleration and hard braking use a lot of fuel. Speeds above about 55 mph (90 km/h) use much more fuel.
- Keep the tires at the right pressure. Soft tires use more fuel.
- Take off extra weight, and roof racks that you do not use.
- Do not idle for long.
- Keep the car serviced. Old air filters and spark plugs can cost you fuel.
What this calculator does not include
- Electric and plug-in hybrid vehicles, which use energy in kilowatt-hours.
- The difference between city and highway driving, unless you measure them separately.
- The other costs of driving, such as insurance, repairs and loss of value.
- Prices in a currency other than the dollar sign. The number is the same in any currency.
This tool gives an estimate for general information only. The scale is a rough guide, and it is not a rating of your vehicle.