Gas Mileage Calculator

Result basis:

Detailed results always include both metric and U.S. fuel-economy readings.

Distance driven:

Use one matched trip, tank, or fill-up interval.

Fuel used:

For a fill-up log, use the pump amount added after resetting or recording the trip distance.

Currency symbol:

Choose the symbol used by your receipt; price zero keeps all cost outputs inactive.

Fuel price:

Price changes cost estimates only; it never changes measured fuel economy.

{{ currency_symbol }} Basis

Tank capacity:

Zero is neutral. The estimate assumes future driving matches the measured interval.

Reference fuel economy:

Zero is neutral. A positive value adds MPG-gap and fuel-delta rows.

U.S. MPG

Measure	Value	Interpretation	Copy
{{ row.measure }}	{{ row.value }}	{{ row.note }}

Scenario	Distance	Fuel needed	Fuel cost	Copy
{{ row.scenario }}	{{ row.distance }}	{{ row.fuel }}	{{ row.cost }}

Gas mileage is trustworthy only when distance and fuel describe the same driving interval. A trip-meter reading paired with the next refill can measure one tank. An odometer difference paired with fuel from another date cannot, because the numerator and denominator no longer belong to the same trip.

Drivers commonly track fuel use to compare commuting with highway travel, estimate the cost of a longer route, notice a sudden change, or check whether a dashboard average matches receipts. One interval is still only a sample. Traffic, speed, short trips, weather, elevation, cargo, tire pressure, idling, roof racks, and heating or cooling loads can move the result without indicating a mechanical fault.

Fuel economy and fuel consumption express the same ratio in opposite directions. Miles per gallon (MPG) and kilometres per litre (km/L) rise when a vehicle travels farther on the same fuel. Litres per 100 kilometres (L/100 km) and U.S. gallons per 100 miles fall when less fuel is used over a fixed distance.

Fuel economy measures and how to compare them
Measure	Reading	Better direction
U.S. MPG	Miles traveled per U.S. gallon	Higher
km/L	Kilometres traveled per litre	Higher
L/100 km	Litres consumed over 100 km	Lower
U.S. gal/100 mi	U.S. gallons consumed over 100 miles	Lower

Equal MPG gains do not always save equal amounts of fuel. Improving from 10 to 15 MPG cuts consumption much more than improving from 30 to 35 MPG over the same distance. Fixed-distance measures such as L/100 km or gal/100 mi make that difference easier to compare because they show fuel directly.

A full-to-full fill-up log usually becomes more stable over several tanks. Pump shutoff timing, vehicle angle, and small refill differences can distort one interval, while totals across multiple matched intervals reduce that noise. Tank capacity is useful for a rough range estimate but does not show usable reserve, gauge behavior, or future driving conditions.

How to Use This Tool:

Use one matched trip, tank, or fill-up interval and choose which economy convention should lead the result.

Choose Metric consumption or U.S. fuel economy as the headline basis. Detailed results include both systems.
Enter the Distance driven and its unit for the measured interval.
Enter the Fuel used over that exact interval in litres or U.S. gallons. For a fill-up log, use the amount added after recording or resetting the trip distance.
Open Advanced only when you want fuel cost, estimated tank range, or a comparison with a positive U.S.-MPG reference. These values add context without changing measured economy.
Compare MPG and L/100 km with the same interval from another tank or route, keeping the measurement method consistent.

Interpreting Results:

Higher MPG and km/L mean more distance per unit of fuel. Lower L/100 km and gal/100 mi mean less fuel over a fixed distance. When a reference is entered, a positive MPG gap means the measured interval beat the reference; a positive fuel delta means it used more fuel than the reference would have used over that same distance.

Cost and range assume the measured fuel-use rate continues. They are forecasts, not new measurements. A large difference from a dashboard or label should be checked across several full-to-full intervals before drawing a conclusion.

Technical Details:

Distance and fuel are converted to kilometres, miles, litres, and U.S. gallons. Every economy and consumption measure is then derived from that same matched interval, so the metric and U.S. readings remain mathematically equivalent.

Formula Core

Distance-per-fuel measures divide distance by fuel. Fixed-distance consumption reverses the ratio and scales it to 100 distance units.

MPG = \frac{d_{mi}}{f_{gal}}, km/L = \frac{d_{km}}{f_{L}}, L/100km = 100 \frac{f_{L}}{d_{km}}

Optional price and capacity values use the same measured rate. Price is normalized to a per-litre value before cost is calculated.

C = f_{L} p_{L}, r_{km} = q_{L} km/L, gap = {MPG}_{measured} - {MPG}_{reference}

Reference fuel use is distance in miles divided by reference MPG. The fuel delta is measured U.S. gallons minus that reference amount, then converted to litres for the metric reading. Forecast rows scale the original distance, fuel, and cost together by 25%, 50%, 75%, 100%, 125%, and 150%, preserving the measured ratio.

Transformation Core

Two exact conversion constants connect the input systems.

Exact distance and fuel-volume conversions
Quantity	Exact conversion	Used for
Distance	1 mile = 1.609344 km	MPG, cost per mile, and range in miles
Fuel volume	1 U.S. gallon = 3.785411784 L	MPG, price per litre, and fuel delta

Inputs must be finite and positive for distance and fuel. Optional fuel price, tank capacity, and reference MPG may be zero, which disables only their related outputs. Calculations keep full numeric precision; displayed values are rounded for readability.

Accuracy Notes:

The arithmetic is deterministic, but the measurement interval can carry more uncertainty than the formula.

Pair fuel and distance from the same interval; do not mix a tank estimate with an unrelated trip reading.
For fill-up tracking, use a consistent full-to-full method and compare several tanks to reduce pump-shutoff noise.
Odometer and pump accuracy, idling, route mix, weather, load, and driving style can all change the result.
Tank range assumes the full entered capacity is usable and future driving matches the measured interval.

Worked Examples:

A matched metric fill-up

A vehicle travels 515 km and the next matched fill-up adds 42.4 L. The interval equals 8.23 L/100 km, 12.15 km/L, and 28.57 U.S. MPG. Those are equivalent readings, not three separate measurements. Entering a fuel price or tank capacity would add cost or range estimates while leaving all three economy values unchanged.

References:

Miles Per Gallon (MPG) Math, U.S. Environmental Protection Agency, updated May 4, 2026.
Appendix C: General Tables of Units of Measurement, National Institute of Standards and Technology, 2026.