Your car's dashboard shows a fuel consumption figure, but it is an estimate from the car's own sensors. The pump and the odometer are what you actually paid for and actually drove. With two full tanks and a calculator, you can work out the real figure yourself.
What you need
- The odometer reading (the total distance on the car) each time you fill up.
- The litres you put in each time. It's on the pump and on the receipt.
- Whether you filled the tank to the top, or only added some.
The method, step by step
- Fill the tank to the top. Keep going until the pump clicks off on its own. Write down the odometer reading. This is your starting point.
- Drive as you normally would. If you top up halfway in between, that's fine. Just write down the litres each time.
- Fill the tank to the top again. Write down the odometer reading and the litres.
Now work out two things:
- Distance: the second odometer reading minus the first.
- Fuel used: every litre you added after the first full tank, including the last fill-up.
Then:
Fuel consumption (L/100km) = fuel used ÷ distance × 100
A worked example
| Fill-up | Odometer | Litres | Filled to the top? |
|---|---|---|---|
| A | 10,000 km | 40 L | Yes |
| B | 10,300 km | 15 L | No |
| C | 10,650 km | 35 L | Yes |
- Distance: 10,650 − 10,000 = 650 km
- Fuel used: 15 + 35 = 50 L
- Fuel consumption: 50 ÷ 650 × 100 = 7.69 L/100km
The mistake most people make
It is tempting to count fill-up A's 40 litres too. Don't. That fuel filled the tank before the measurement started. You burned it on the drive to fill-up A, not between A and C.
Counting it gives 90 ÷ 650 × 100 = 13.85 L/100km. That's almost double the real figure, and it's the most common error in fuel logs, both in spreadsheets and in apps.
Converting to km/L or MPG
Different countries measure fuel economy differently. With L/100km, a lower number is better. With km/L and MPG, a higher number is better. Mixing these up is the easiest way to get a comparison backwards.
- km per litre = 100 ÷ L/100km
- US miles per gallon = 235.2 ÷ L/100km
- UK miles per gallon = 282.5 ÷ L/100km (a UK gallon is bigger than a US one)
The example above, 7.69 L/100km, is 13.0 km/L, 30.6 US MPG or 36.7 UK MPG. Some common values:
| L/100km | km/L | US MPG | UK MPG |
|---|---|---|---|
| 5 | 20.0 | 47.0 | 56.5 |
| 6 | 16.7 | 39.2 | 47.1 |
| 7 | 14.3 | 33.6 | 40.4 |
| 8 | 12.5 | 29.4 | 35.3 |
| 10 | 10.0 | 23.5 | 28.2 |
Tips for a number you can trust
- Use the same pump click each time. Stop when the pump clicks off the first time; don't squeeze in extra. "Full" then means the same thing every time.
- Average several tanks. One tank can be skewed by a long motorway drive or a week of traffic jams. Three or four tanks give you a steadier figure.
- Never skip a receipt. A missed top-up means missing litres, and the car will look more economical than it is.
What about electric cars?
For an electric car, the unit is kWh per 100 km: kilowatt-hours, the same unit as on your electricity bill. You rarely charge to exactly 100%, so full-to-full doesn't work well. Instead, add up every kWh you charged over a period, such as a month, and divide by the distance you drove in it.
Say you charged 20, 18 and 22 kWh in a month and drove 400 km: 60 ÷ 400 × 100 = 15.0 kWh/100km.
The charger counts the energy that went in, including a little lost as heat while charging. So this figure usually reads a bit higher than the one on the car's screen. It's also the one you paid for.
How OdoTick does it
OdoTick uses exactly this method. Each fill-up has a Full tank switch. After two full tanks, it works out your fuel consumption and leaves out the first tank's litres, as above. Until then it shows a dash rather than a guess. For electric cars, it uses the last 30 days of charging.