Compare fuel used over one distance
For a trip record, divide total litres by total kilometres and multiply by 100. Convert that consumption using the correct gallon convention, keeping the actual totals for review.
The following inputs stay in litres/100 km and the results use miles/us gallon. They illustrate different magnitudes for this measurement. Retain the specific conditions described above when replacing an example with your own value.
The mpg display falls as litres per 100 km rises. This is the reciprocal response expected from economy and consumption. Compare the actual fuel used over an agreed distance when evaluating a saving, rather than comparing percentages from opposite representations.
Retain each trip’s fuel and distance totals so the combined figure can be recalculated instead of averaging already-rounded economy numbers.
7 L/100km in miles/us gallon
7 L/100km = 33.6020833333 mpg-US
US mpg = 235.214583333 ÷ (L/100 km)For the starting consumption-to-US-economy example, 235.214583333333 ÷ 7 = 33.6020833333 mpg-US. This is a reciprocal calculation. Doubling consumption halves economy rather than doubling it.
The fuel relationship uses the named gallon convention and positive inputs. A zero consumption value cannot be used in the reciprocal equation. To compare journeys, retain the distance and fuel totals, then calculate the appropriate consumption or economy figure. Swapping the direction changes the representation of the same supplied fuel relationship.
Reversing the unrounded 33.6020833333 mpg-US result returns 7 L/100km. Use this as a direction check. A consistent reverse calculation cannot resolve averaging mpg readings without weighting or improve the original observation.
Compare the same distance and the same gallon
Two trips can return different economy figures even when their combined fuel use is easy to measure. Averaging the two mpg numbers without weighting can give the wrong overall result. Litres per 100 km and miles per gallon are reciprocal ways of reporting distance and fuel, so the unit convention and aggregation method both matter.
For a 600 km trip at 7 L/100 km, the model predicts 42 L of fuel. At 8 L/100 km it predicts 48 L. The difference is 6 L for this distance. Converting consumption to mpg can help a reader, but the underlying litres used remain easier to compare when estimating the trip total.
Consumption in L/100 km and economy in mpg move in opposite directions. Halving L/100 km doubles mpg under the same gallon convention. A percentage improvement in one representation is therefore not the same percentage change in the other. Compare fuel used over a common distance when you want a transparent saving calculation.
US and Imperial gallons differ, so identical mpg numbers do not indicate identical consumption. Identify the market and source convention rather than assuming the vehicle's location is enough. Actual consumption depends on conditions and the measurement method; a specification is not a guarantee for one journey. Fuel price, trip distance and consumption should each appear as separate inputs in a cost estimate.
600 km at 7 L/100 km uses 42 L; at 8 L/100 km it uses 48 L.
What this fuel economy comparison can tell you
Useful capabilities
Retain each trip’s fuel and distance totals so the combined figure can be recalculated instead of averaging already-rounded economy numbers.
The listed units express the same supplied fuel economy quantity. The 7 L/100km → 33.6020833333 mpg-US example makes the selected convention visible before it is used in a longer calculation.
Important limits
Consumption and economy are reciprocals. Values must be positive; US and Imperial gallons differ.
Combine the actual distances and fuel volumes before calculating the overall economy. Use the reciprocal formula; the direction of improvement is reversed.
Averaging mpg readings without weighting
Combine the actual distances and fuel volumes before calculating the overall economy.
Assuming a fixed multiplier connects L/100 km and mpg
Use the reciprocal formula; the direction of improvement is reversed.
Averaging mpg readings directly can misrepresent combined trip economy. Add the relevant distances and fuel quantities first, then calculate the ratio. US and Imperial gallons use different definitions, so keep that label with the result. If estimating trip fuel, preserve the expected distance and consumption separately. A specification value alone cannot account for one journey’s load, route or driving conditions.
As a controlled uncertainty example, suppose the original 7 L/100km record had a stated ±1% input interval. Its endpoints would be 6.93 L/100km and 7.07 L/100km. Converting those endpoints gives an output interval from 33.2693894389 mpg-US to 33.9414983165 mpg-US. The reciprocal relationship reverses endpoint order and produces a non-symmetric interval around the starting result. This chosen interval illustrates the arithmetic and is not an accuracy claim for your measurement.
Consumption and economy move opposite ways
Fuel consumption and fuel economy describe the same relationship in opposite ways. Litres per 100 km reports fuel used for a distance; km/L and mpg report distance travelled per fuel volume. A lower consumption figure therefore corresponds to a higher economy figure.
US mpg = 235.214583333333 ÷ (L/100 km), while Imperial mpg uses approximately 282.480936331822 in the numerator. This reciprocal relationship means doubling L/100 km halves mpg. The US and Imperial gallon definitions differ, so a bare mpg label is incomplete. Values must be positive; zero consumption cannot be inverted into a finite economy.
For a combined record, sum the actual fuel volumes and distances first. Calculate consumption from those totals, then convert to the chosen economy unit. US and Imperial gallons differ, and consumption changes in the opposite direction from economy: higher L/100 km means lower mpg. Driving-cycle ratings and measured trip averages should remain distinct.
Retain each trip’s fuel and distance totals so the combined figure can be recalculated instead of averaging already-rounded economy numbers.
Questions about fuel economy
Why is zero rejected?
The listed units involve division by the supplied value. Zero would produce an undefined or infinite reciprocal result rather than a usable finite conversion.
Can I compare US and UK vehicle listings?
Yes, once the mpg convention is known and the test conditions are comparable. Unit conversion cannot eliminate differences between driving cycles, weather, loading or measurement methods.
Fuel Economy units, side by side
The values below all describe 7 L/100km. They provide a comparison of unit sizes for fuel economy; they are not separate measurements. Read the complete symbol and convention before copying a number.
| Unit | Symbol | Equivalent |
|---|---|---|
| Litres/100 km | L/100km | 7 L/100km |
| Kilometres/litre | km/L | 14.2857142857 km/L |
| Miles/US gallon | mpg-US | 33.6020833333 mpg-US |
| Miles/Imperial gallon | mpg-UK | 40.354419476 mpg-UK |
Keep the original litres/100 km record alongside the converted miles/us gallon display. The unit list uses up to twelve significant digits for arithmetic readability, while the measurement precision still comes from its source.
Before using this fuel economy result
- Identify whether mpg uses a US or Imperial gallon.
- Separate a laboratory rating from a measured trip average.
- For combined trips, use total fuel and total distance rather than a simple mean of mpg values.
Keep the original fuel economy value, its unit and the relevant conditions in the record. A copied answer of 33.6020833333 mpg-US is interpretable only when those details remain available. State which of the measurement checks above you verified before using the value in another calculation.