Fuel Consumption Calculator – MPG & L/100 km

Continue from measured fuel economy

Use the completed fill-to-fill result to price a journey, convert units or learn why different energy carriers cannot be compared by the raw number.

Measure fuel consumption before calculating fuel cost

This calculator answers an observed-performance question: how much fuel did the vehicle use for the distance already travelled? It deliberately does not ask for a fuel price or destination. Once you have a credible consumption figure, you can use it in a trip-cost or total-ownership calculation without confusing measured economy with a planned expense.

L/100 km and MPG express the same physical relationship in opposite directions. L/100 km states the fuel needed for a fixed distance, so a lower figure is better. MPG states the distance obtained from a fixed amount of fuel, so a higher figure is better. That inversion is why a reduction from 10 to 8 L/100 km is not equivalent to adding a fixed number of MPG at every starting point.

The result is an estimate of the measured interval, not a permanent property of the vehicle. Traffic, speed, temperature, load, tyres, elevation, short trips and auxiliary use can all move the next fill-up.

Use the fill-to-fill method for a repeatable result

1. Fill the tank to a consistent stopping point and record the odometer or reset a trip counter.
2. Drive a useful interval. A longer distance normally reduces the percentage effect of pump shut-off and rounding.
3. Refill at a level comparable with the first fill, ideally with the vehicle on similar ground.
4. Enter the distance travelled and the fuel volume shown on the second receipt.
5. Repeat across several tanks and calculate a combined total before treating the number as a long-term average.

Do not divide one refill by the car's lifetime odometer. The distance and fuel volume must cover the same interval. If the first tank was not full but the second was, some of the second fill merely corrects the starting level and distorts the result.

Receipts and trip readings are worth keeping. A sudden change is easier to investigate when you can distinguish one unusual journey from a trend across several measurements.

Fuel-consumption formulas and unit conversions

For metric inputs, the core formula is:

Distance per litre reverses the fraction:

For a direct MPG measurement, divide miles by gallons. The gallon definition matters: 1 US gal = 3.785411784 L, while 1 imperial gal = 4.54609 L. The calculator converts every input to kilometres and litres first, then reports all four common formats from the same unrounded value.

How the reference comparison works

The optional reference stays in L/100 km so the comparison has one unambiguous basis. It can be a combined manufacturer figure, the car's previous long-term average, another vehicle measured on a comparable route or a target after a maintenance change.

A positive difference means the measured interval used more fuel than the reference. A negative difference means it used less. The calculator also converts the gap into litres over 10,000 km, which makes a small-looking decimal difference easier to interpret.

A comparison is only as fair as its conditions. A laboratory or certification cycle, a winter commute and a warm long-distance trip are not interchangeable test environments.

Consumption sensitivity over the same distance

The table keeps your measured distance and tank size constant and shows a 10% improvement and deterioration around the measured consumption. It is a sensitivity check, not a forecast.

Tank range is a planning estimate, not a target

Range is calculated from the entered usable capacity and the measured L/100 km. The nominal tank size in a specification may not equal the amount you can safely use before refuelling. Reserve strategy, gauge calibration, gradients and changing consumption all matter.

A single motorway fill-up may overstate urban range; a cold short-trip measurement may understate a later summer journey. Use a conservative consumption value for route planning and keep an appropriate margin for station availability, weather and detours.

Never plan to consume every theoretical litre. The range output is designed to compare scenarios, not to encourage driving until the engine stops.

Worked measurements

Practice: combine several fill-ups correctly

Three consecutive full-tank intervals cover 540 km with 36 L, 610 km with 41 L and 575 km with 37 L. Find the combined consumption. Do not average the three L/100 km figures unless the distances are identical.

Common errors that can move the answer

• Selecting US gallons for an imperial-gallon receipt, or the reverse.
• Using the fuel gauge instead of a measured pump volume.
• Mixing kilometres with miles or litres with gallons without conversion.
• Comparing a short winter trip with a warm official combined figure as if conditions were identical.
• Averaging tank-by-tank consumption values without weighting for different distances.
• Entering total odometer distance instead of distance since the relevant fill.
• Assuming nominal tank capacity is fully usable.
• Rounding MPG before converting back to L/100 km.

What this result can and cannot diagnose

A sustained increase in consumption can be a reason to review tyre pressure, route mix, seasonal conditions, load and service history. It does not by itself identify a mechanical fault. One unusual fill can also result from pump cut-off, parking angle or an incomplete previous fill.

Compare like with like and look for a pattern over several tanks. If fuel use changes abruptly alongside warning lights, fuel smell, leakage, poor running or another safety concern, stop treating it as a spreadsheet problem and have the vehicle inspected appropriately.

Next in the Automotive build will be the Trip Fuel Cost Calculator, which will take a known consumption figure and turn it into fuel required, journey cost, round-trip cost and per-person sharing.