Fuel consumption calculator

How this is calculated

Miles per gallon is the miles you drove divided by the gallons you added to refill the tank. L/100km is the same fill-up expressed in litres per 100 kilometres: 100 × gallons × 3.785412 ÷ (miles × 1.609344). The two constants are from the NIST Guide to the SI, Appendix B. To find the fuel needed for a distance, divide the distance by your mpg.

mpg = miles ÷ gallons   and   gallons needed = distance ÷ mpg

The method, the constants and the limits are written out in how fuel cost is calculated.

How to measure mpg accurately

1. Fill the tank until the pump clicks off the first time, then note the trip meter or odometer and reset the trip meter to zero.
2. Drive as you normally do until the tank is at least half empty, or lower if you can.
3. Fill again at the same pump if you can, stopping at the first click-off, and write down the gallons the pump shows.
4. Enter the miles on the trip meter and the gallons added.

Topping off after the click, different pumps and a different parking angle can all change how much fuel goes in, so a single tank gives a rough figure. Averaging several tanks smooths this out. The dashboard’s own mpg readout is useful for a comparison, but a tank-to-tank figure is based on fuel you actually bought.

Keeping a log

Note the date, miles and gallons at every fill-up and work out each tank on its own. Over a few months you will see how your mpg changes with the seasons, with short trips and with highway driving. A sudden drop that lasts can be worth mentioning to a mechanic. Use your average mpg in the fuel cost calculator and the trip gas calculator for better estimates.

To convert a figure between the two systems on its own, use the mpg to L/100km converter. For the full cost of owning a car per mile, see the cost per mile calculator. The method behind fuel estimates is in how fuel cost is calculated, and how much gas you will use on a trip applies it to a drive.