Why use this calculator
Your own measured economy is the most useful number you can know about your car. It tells you what trips really cost, whether the car is running properly and how far it is from the figure on the showroom label. A sudden rise in consumption is often the first sign of a problem such as under-inflated tyres, a dragging brake or a failing sensor, long before a warning light appears.
The trip computer helps, but it is commonly optimistic by a few per cent, so the fill-to-fill method is the reference. This calculator does the division and the conversions, including the trap that US and UK gallons differ, so a British 40 mpg is only about 33 mpg American.
It also compares your figure with the official label, since lab test figures typically overstate what you will see on the road, and flags a gap large enough to investigate.
How to use it
- Pick your units: metric, US or UK.
- Fill the tank to the brim and reset the trip meter, drive normally, then fill to the brim again. Enter the distance shown on the trip meter.
- Enter the fuel it took to refill from the pump receipt, in litres or US gallons.
- Optionally enter the price per litre or per gallon from the receipt to see the running cost.
- Optionally enter the official label figure from the fuel label, window sticker or fueleconomy.gov to see how your driving compares with the lab test.
What each result means
- Fuel economy
- Your measured economy in the units you chose. For L/100km lower is better; for mpg higher is better.
- Litres per 100 km
- The metric figure used on Australian, New Zealand and European labels, so you can compare your car with others.
- US miles per gallon
- The same economy in US mpg on the 3.785 litre gallon, as used on US window stickers and fueleconomy.gov.
- UK miles per gallon
- The same economy on the 4.546 litre imperial gallon, which reads about 20 per cent higher than US mpg.
- Against the official figure
- How much more or less fuel you use than the lab test figure. A gap of 10 to 25 per cent is normal.
- Fuel cost of the gap to the label
- The dollars this tank cost above, or below, what the same distance would have cost at the official label figure, so the percentage gap becomes money you can act on.
- Fuel cost for this tank
- What the refill cost at the price you entered, the actual spend for the distance you measured.
- Running cost per distance
- Fuel spend for each kilometre or mile, the fuel part of your running costs only.
The formula
L/100km = litres / kilometres x 100
US mpg = 235.215 / L/100km
UK mpg = 282.481 / L/100km
Note that L/100km and mpg are inversely related, so they move in opposite directions.
Fuel cost = litres x price per litre
Running cost = fuel cost / distance
Gap against the label = (your L/100km / label L/100km - 1) x 100 per cent
How this number is used in practice
The fill-to-fill method is only as good as the fills. Use the same pump if possible, stop at the first automatic click each time, and average across three or four tanks to smooth out a single odd trip. Trip computers typically read a few per cent optimistic, and the odometer itself can be out by a couple of per cent, especially after a change of tyre size.
Because L/100km and mpg are inverses, improvements do not look the same on both scales. Going from 16 to 12 L/100km saves 4 litres every 100 km, while going from 6 to 4 L/100km saves only 2, even though the mpg numbers for the second change look bigger.
That is why replacing a thirsty four-wheel drive usually saves more fuel in dollars than swapping an already economical small car for a hybrid. For comparing running costs, L/100km is the honest scale.
Official figures are lab results. US EPA ratings were revised in 2008 to include faster driving, air conditioning and cold starts, so the gap for American cars is smaller. Australian labels and older European figures use gentler test cycles, and independent real-world testing has repeatedly found cars using noticeably more than their label. Towing, short trips, idling and speeds over about 100 km/h widen the gap.
Worked examples
560 km on 45 litres at $1.90 a litre, label figure 7 L/100km
8.04 L/100km (29.3 US mpg), 15% more fuel than the label, $85.50 for the tank
A US driver covers 350 miles on 12 gallons at $3.50, label 30 mpg
29.2 US mpg, 3% more fuel than the label, $0.12 per mile
A UK car does 420 miles on 48 litres, label 52 UK mpg
39.8 UK mpg (7.10 L/100km), 31% more fuel than the label
When people use it
- Checking your real economy against the showroom label figure
- Spotting a mechanical problem from a sudden rise in consumption
- Converting a UK or US mpg figure into L/100km for an Australian comparison
- Measuring the effect of a roof rack, towing or a change of tyres
- Working out an accurate cost per kilometre for reimbursement
Tips
- Average over three or four tanks. A single tank can be skewed by one long highway run or a week of short trips.
- Fill at the same pump and stop at the first click for consistent fills.
- Check tyre pressures monthly. Low pressure is the most common fixable cause of poor economy.
- Compare L/100km rather than mpg when weighing up savings. It scales with fuel actually used.
- A jump of more than 15 per cent with no change in driving is worth a mechanic's look.
- Short trips of a few kilometres use far more fuel per kilometre because the engine never warms up.
What it assumes, and where it stops being accurate
- Both fills are to the same level, normally the first automatic click. A partial fill makes the result wrong.
- The trip meter distance is accurate. Odometers can be a few per cent out, more with non-standard tyres.
- The label comparison uses a single combined figure. Urban and highway label figures differ widely.
- US gallons are 3.785411784 litres and UK gallons 4.54609 litres.
- Fuel cost uses a single price for the whole tank.
- Plug-in hybrids that also used grid electricity will show an unrealistically low fuel figure.
Terms used on this page
- Fill-to-fill method
- Measuring economy by filling the tank, driving, and dividing the refill amount by the distance.
- US gallon
- 3.785 litres. Used for US mpg figures and US fuel prices.
- Imperial gallon
- 4.546 litres. Used for UK mpg, although UK fuel is sold by the litre.
- Combined figure
- An official economy figure that blends urban and highway test results.
Questions people ask
- How do I measure my fuel economy accurately?
- Fill the tank until the pump clicks off, reset the trip meter, drive normally, then fill to the click again. Divide the litres by the kilometres and multiply by 100 for L/100km, or divide miles by gallons for mpg. Average several tanks for the best figure.
- Why do L/100km and mpg go in opposite directions?
- L/100km measures fuel used for a fixed distance, so lower is better. mpg measures distance travelled on a fixed amount of fuel, so higher is better. They are reciprocals: US mpg is 235.215 divided by L/100km, and UK mpg is 282.481 divided by L/100km.
- What is a good fuel economy?
- For a petrol small car, 5 to 7 L/100km is good. Mid-size cars run about 7 to 9, large SUVs and utes 9 to 13, and hybrids 4 to 5. In US terms that is roughly 34 to 47 mpg for small cars and 18 to 26 mpg for large SUVs.
- Why is my car using more fuel than the label says?
- Lab tests are gentler than real driving. Speed, short trips, air conditioning, cold weather, load, towing and hilly roads all add fuel. A 10 to 25 per cent gap is normal. A gap above about 30 per cent points to tyres, a mechanical fault or unusual driving conditions.
- Is my trip computer accurate?
- Usually close but slightly optimistic, often by 2 to 5 per cent, because it estimates fuel from injector timing rather than measuring it. The fill-to-fill method is the reference. Compare the two over a few tanks to learn your car's bias.
Use this with
Fuel Efficiency Calculator answers one part of the question. These pick up where it stops: Fuel Cost Calculator, Road Trip Planner Calculator, Car Running Cost Calculator and Tyre Size Comparison Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- US Department of Energy and EPA: fueleconomy.gov. US ratings and the 2008 revision of test procedures
- Australian Government Green Vehicle Guide. Australian fuel consumption label figures
- The International Council on Clean Transportation. Research on the gap between lab and real-world fuel consumption
Fuel Efficiency Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.