Calculate gas mileage from odometer readings or trip distance, convert US and Imperial MPG, and estimate refill, per-mile, and annual fuel cost.
Calculator
Gas Mileage Results
Compare US and Imperial MPG, metric consumption, fill cost, annual fuel use, and an optional EPA reference.
Measured Efficiency and Annual Fuel
Mileage and Fuel Cost Details
| Measurement | Value | Calculation note |
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Calculate real-world gas mileage
Use two full-tank fill-ups for the most useful result. At the first fill-up, record the odometer or reset the trip meter. Drive normally. At the next fill-up, refill to the same point and record the new odometer and the fuel added. The fuel pumped during the second fill-up approximates the fuel used between readings.
Select Odometer readings to let the calculator subtract the earlier reading from the current reading. Select Trip distance when the vehicle already reports the distance traveled. Choose miles or kilometers independently from US gallons, Imperial gallons, or liters; the calculator normalizes both measurements before reporting every common efficiency format.
distance traveled = current odometer - previous odometer
US MPG = miles traveled / US gallons added
MPG calculation example
A previous odometer reading of 12,000 miles and a current reading of 12,300 miles produce a 300-mile trip. If the refill takes 12 US gallons, fuel economy is 25 US MPG. That same physical efficiency is about 30.02 Imperial MPG, 9.41 L/100 km, or 10.63 km/L.
At $3.50 per US gallon, the fill costs $42 and the measured fuel cost is $0.14 per mile. If that mileage and price remain representative for 12,000 miles of annual driving, the fuel-only annual estimate is $1,680. The annual figure excludes maintenance, insurance, depreciation, tolls, parking, and electricity used by plug-in vehicles.
US MPG, Imperial MPG, km/L, and L/100 km
US MPG and Imperial MPG both measure distance per gallon, but the gallon sizes differ. One Imperial gallon is larger than one US gallon, so the Imperial MPG number is about 20% higher for the same physical fuel economy. Always identify the gallon definition before comparing a US vehicle result with a UK publication.
Kilometers per liter is another distance-per-volume measure, so a higher number is better. Liters per 100 kilometers measures consumption instead: it reports how much fuel is needed to travel 100 km, so a lower number is better.
L/100 km = 235.214583 / US MPG
Imperial MPG = US MPG x 1.2009499
Fuel cost per mile and annual fuel cost
Enter the pump price per selected fuel unit to calculate the second fill-up cost. Cost per mile divides that fill cost by the normalized mileage. Annual fuel cost applies the same measured cost per mile to the annual distance you enter. Currency changes the symbol only; the calculator does not convert exchange rates.
fill cost = fuel added x price per selected fuel unit
cost per mile = fill cost / miles traveled
Fuel prices and driving conditions change, so treat annual cost as a planning estimate. For a route-specific calculation with one-way, round-trip, multiple-trip, and cost-sharing options, use the trip fuel cost calculator.
Compare measured MPG with an EPA estimate
The optional reference field accepts a US MPG value. Enter the combined rating for the exact model and configuration to see the percentage difference between the measured fill-up and that reference. Use the official fueleconomy.gov vehicle comparison to find US ratings. A single tank is not enough to diagnose a vehicle, because weather, route mix, speed, idling, towing, payload, and fill level can materially change the result.
How to improve measurement accuracy
- Fill at the same station and pump when practical, with the vehicle on level ground.
- Stop at the same automatic nozzle shutoff rather than topping off.
- Use the actual pump volume from the second fill-up, not tank capacity or a dashboard estimate.
- Record several consecutive tanks and divide total distance by total fuel for a more stable average.
- Note unusual towing, idling, weather, traffic, or tire-pressure conditions before comparing tanks.
The US Department of Energy's Consumer Guide to Fuel Economy explains that driving behavior, vehicle condition, and the environment can significantly affect real-world mileage.
Why measured MPG can differ from the dashboard
A trip computer estimates fuel use from vehicle sensors and software, while a fill-to-fill result uses odometer distance and pump volume. The two methods may cover different time periods and each has measurement error. Small differences are normal. A persistent, material change across several comparable tanks can justify checking tire pressure, maintenance needs, seasonal fuel, route changes, or driving behavior.
Frequently asked questions
Should I use tank capacity as fuel used?
No. Use the amount dispensed at the second full refill. Tank capacity does not show how much fuel was actually consumed during the measured distance.
Can I enter kilometers and liters?
Yes. Select kilometers and liters. Results still include US MPG, Imperial MPG, L/100 km, and km/L.
Can I mix miles and liters?
Yes. Distance and fuel units are selected independently, and the calculator converts them before calculating efficiency.
Is MPGe the same as MPG?
No. MPGe is an energy-equivalent comparison for electricity and alternative fuels. This calculator uses liquid-fuel volume.
How many fill-ups should I average?
There is no required number, but several consecutive full-tank intervals reduce the effect of small fill-level differences. Add the distances together and divide by the total fuel added.