The tank-to-tank method is the only measurement you control
Fuel economy is one division: distance covered divided by fuel consumed. What makes it awkward is that neither quantity is easy to observe precisely. You cannot see how much fuel is in the tank, and the trip computer's instantaneous figure is an inference from injector pulse widths, not a measurement of liquid.
The tank-to-tank method removes the guesswork by using the pump as the measuring instrument. Fill the tank until the nozzle clicks off, reset the trip odometer, drive, and refill at the same pump style until it clicks off again. The gallons on the display are exactly the gallons you burned, and the trip odometer is exactly the distance. Divide one by the other.
Three details make the difference between a measurement and a number. Use the same fill point every time — first click, not topping off to the brim — because the whole method depends on both fills ending at the same liquid level. Reset the trip odometer at the fill, not at the start of the drive. And average at least three tanks, because a single tank contains one particular mix of traffic, weather and terrain, and the fill-level error at each end is a fixed quantity spread over a variable distance.
Once you have a number, it is worth knowing how the official figures were produced for comparison. US window-sticker MPG comes from the EPA's test procedures in 40 CFR Part 600 — five standardised dynamometer cycles covering city, highway, high-speed, air-conditioned and cold-weather operation, combined with weighting factors. They are repeatable and comparable between vehicles, and they are not your commute.
Four units, two of them reciprocals of each other
US MPG is miles per US gallon of 3.785411784 litres. Imperial MPG is miles per Imperial gallon of 4.54609 litres. Because the Imperial gallon is larger, the same car scores about 20% higher in Imperial MPG — the exact factor is 1.20095. A car advertised at 48 MPG in the UK does 40 MPG in the US, and neither figure is wrong.
km/L is the direct metric analogue: multiply US MPG by 1.609344 ÷ 3.785411784 = 0.4251437. L/100 km, used across Europe, inverts the whole idea — it measures consumption rather than economy, so lower is better. The conversion is 235.2146 ÷ MPG, where the constant is simply 100 × 3.785411784 ÷ 1.609344.
That inversion matters more than it sounds. Because fuel used is proportional to 1/MPG rather than to MPG, equal steps in MPG are not equal savings. Going from 15 to 20 MPG saves 1.67 gallons per hundred miles. Going from 40 to 50 MPG — a larger step, and a much harder engineering problem — saves 0.50 gallons per hundred miles. If you own two vehicles and can only improve one, improving the thirsty one saves more fuel, which is exactly why the L/100 km convention exists: it makes the saving visible directly.
Cost per mile folds price into the picture: price ÷ MPG. It is the number to use when comparing against an electric vehicle, where the equivalent figure comes out of the EV charging cost calculator as dollars per mile on the same basis.
Worked example: 300 miles on 12 gallons at $3.45
You fill up, reset the trip odometer, drive 300 miles, and it takes 12.0 gallons to refill to the same click. Fuel is $3.45 a gallon and you drive 12,000 miles a year.
- Fuel economy. 300 ÷ 12.0 = 25.00 US MPG.
- Imperial MPG. 25.00 × 1.20095 = 30.02 Imperial MPG.
- km per litre. 25.00 × 0.4251437 = 10.63 km/L.
- L per 100 km. 235.2146 ÷ 25.00 = 9.41 L/100 km. Check it against the km/L figure: 100 ÷ 10.63 = 9.41. They agree, as they must.
- Cost of this fill. 12.0 × $3.45 = $41.40.
- Cost per mile. $3.45 ÷ 25.00 = $0.1380, or $13.80 per hundred miles.
- Annual fuel cost. 12,000 × $0.1380 = $1,656.00.
Now suppose you improve to 30 MPG by keeping the tires at pressure and easing off the highway speed. Cost per mile falls to $3.45 ÷ 30 = $0.1150 and the annual bill to $1,380 — a saving of $276 a year. Improving further from 30 to 35 MPG saves only another $197, despite being the same five-MPG step, because the relationship is reciprocal.
How to read the result
Compare against your own history, not against the sticker. The useful signal in a fuel-economy figure is change over time in the same vehicle on the same routes. A number that has drifted down 10% over a few months points at something specific: tire pressure, a dragging brake, a clogged air filter, a failing thermostat holding the engine below operating temperature, or simply a change in how or where you are driving.
Expect the trip computer to read optimistic. Most read a little high, and some read several percent high. If you trust the tank-to-tank number, note the offset once and mentally apply it thereafter rather than re-measuring every tank.
Watch for anything that changed the odometer. A taller tire makes the odometer under-count miles, which makes calculated MPG read low; a shorter tire does the opposite. If you have changed tire size, quantify the error with the tire size comparison calculator and correct the distance before dividing.
Treat highway speed as the biggest single lever you control. Aerodynamic drag rises with the square of speed and the power to overcome it rises with the cube, so the last increment of speed is disproportionately expensive. Steady-state cruising, smooth inputs, correct tire pressure and removing roof boxes when they are not in use all show up in the tank-to-tank number.
Do not read fuel cost as the cost of driving. It is typically a minority of the total. Depreciation is usually larger over any normal holding period — size it with the vehicle depreciation calculator before assuming that a more economical car saves you money overall.
MPG, L/100 km and the cost of 100 miles
| US MPG | Imperial MPG | L/100 km | km/L | 100 mi at $3.00 | 100 mi at $3.45 | 100 mi at $4.50 |
|---|---|---|---|---|---|---|
| 15 | 18.01 | 15.68 | 6.38 | $20.00 | $23.00 | $30.00 |
| 20 | 24.02 | 11.76 | 8.50 | $15.00 | $17.25 | $22.50 |
| 25 | 30.02 | 9.41 | 10.63 | $12.00 | $13.80 | $18.00 |
| 30 | 36.03 | 7.84 | 12.75 | $10.00 | $11.50 | $15.00 |
| 35 | 42.03 | 6.72 | 14.88 | $8.57 | $9.86 | $12.86 |
| 40 | 48.04 | 5.88 | 17.01 | $7.50 | $8.63 | $11.25 |
| 45 | 54.04 | 5.23 | 19.13 | $6.67 | $7.67 | $10.00 |
| 50 | 60.05 | 4.70 | 21.26 | $6.00 | $6.90 | $9.00 |
Read the cost columns down and the reciprocal effect is obvious: at $3.45 a gallon, moving from 15 to 20 MPG saves $5.75 per hundred miles, while moving from 45 to 50 MPG saves $0.77. Both are five-MPG improvements.
What makes a tank-to-tank figure wrong
- Filling to different levels. The method assumes both fills end at the same liquid level. Topping off past the first click on one fill and not the other injects an error of a litre or more straight into the divisor.
- Forgetting to reset the trip odometer. An unreset odometer inflates the distance and therefore the MPG. If you forget, use the difference between two total-odometer readings instead.
- Using a single tank. One tank is one sample of one mix of driving. Three consecutive tanks averaged is a far better estimate, and it dilutes the fill-level error.
- Mixing units. Entering litres against miles, or Imperial gallons against a US price, produces a plausible-looking number that is wrong by 20% or more. Use the unit selectors rather than converting in your head.
- Ignoring an odometer error. A non-standard tire size shifts every distance the vehicle records, and therefore every economy figure derived from it.
- Comparing winter tanks with summer tanks. Cold air is denser, engines run rich until warm, winter fuel blends carry slightly less energy per gallon, and short cold trips never reach operating temperature. A seasonal drop of several percent is normal and is not a fault.
How your number relates to the official ones
The EPA figure on a US window sticker comes from laboratory dynamometer testing under 40 CFR Part 600, using five cycles: the city and highway cycles that have been in use for decades, plus a high-speed cycle, an air-conditioning cycle and a cold-temperature cycle added to make the published numbers closer to real driving. The results are combined and adjusted, and the combined figure is weighted 55% city and 45% highway.
The purpose of that process is comparability, not prediction. Every vehicle is measured the same way, so the ranking between vehicles is meaningful even where the absolute numbers do not match your experience. Your own tank-to-tank figure is the prediction; the sticker is the benchmark.
Europe and much of the rest of the world now use WLTP, the Worldwide Harmonised Light Vehicle Test Procedure, quoted in L/100 km. It replaced the older NEDC cycle, which was widely criticised for producing figures well below real consumption. Comparing a WLTP L/100 km figure with an EPA MPG figure requires converting units and accepting that the test cycles differ, so the comparison is indicative at best.
For trip-level planning rather than long-run averages, the more useful calculation is total fuel for a known distance — including the substantial penalty from towing or a roof box. The road trip fuel cost calculator does that arithmetic and works out how many fuel stops the trip needs.
Key terms
- Fuel economy
- Distance per unit of fuel — MPG or km/L. Higher is better. It is the reciprocal of fuel consumption.
- Fuel consumption
- Fuel per unit of distance — L/100 km. Lower is better. It scales linearly with the fuel you actually buy, which is why it makes savings easier to see.
- Imperial gallon
- 4.54609 litres, used in the UK and some Commonwealth countries. It is 20.095% larger than the US gallon of 3.785411784 litres.
- Tank-to-tank method
- Measuring economy by dividing trip-odometer distance by the fuel needed to refill to the same level. The pump is the measuring instrument.
- Cost per mile
- Fuel price divided by fuel economy. The single most useful figure for comparing running costs across vehicles and across fuel types.