How to use the mpg calculator
- Choose units: miles and gallons, or kilometers and liters.
- Fill the tank and enter the odometer reading as your starting point.
- Each time you refill to full, add a fill-up with the odometer at that moment and the fuel added. Optionally enter the price per gallon or liter. Use the Add a fill-up button as needed.
- Read the combined fuel economy, the best and worst fill-up, the distance and fuel tracked and the cost per mile or kilometer.
Formula
Distance for a fill-up = its odometer reading - the previous reading
MPG = miles / gallons; L/100 km = liters / kilometers x 100
Combined economy = total distance / total fuel (not the average of the individual figures)
Cost per mile = total fuel cost / total distance
Conversions used: 1 mile = 1.609344 km and 1 US gallon = 3.785411784 liters. The fuel at your first fill-up is not counted, because it refilled the tank for distance driven before the log began.
Worked example: four fill-ups
The starting odometer is 41,200. The next four fill-ups are at 41,468 (9.8 gal), 41,731 (9.6 gal), 42,012 (10.4 gal) and 42,270 (9.4 gal). The distances are 268, 263, 281 and 258 miles, or 1,070 miles in total, on 39.2 gallons.
The combined economy is 1,070 / 39.2 = 27.3 MPG, which is 8.62 L/100 km. The individual fill-ups are 27.35, 27.40, 27.02 and 27.45 MPG, so the best is the fourth and the worst the third.
With prices of $3.42, $3.55, $3.39 and $3.61 per gallon, fuel cost $136.79, which is $0.13 per mile or $12.78 per 100 miles. In metric units, 1,070 km on 39.2 liters is 3.66 L/100 km.
Why track several fill-ups
A single tank gives a noisy answer. Pump click-off points differ, temperatures change, and a trip with heavy traffic or a hilly route skews one number. Looking at several fill-ups smooths that out. The combined figure divides total distance by total fuel, which weights each tank by how far it went. The table lets you spot an unusual tank, which can point to a change in driving, tire pressure or a mechanical issue worth checking.
Getting consistent numbers
Fill to the same point each time, such as the first automatic shutoff, and use the same pump when you can. Record the odometer and the fuel amount at every fill, not only when convenient. Skipping a fill-up is fine in this tool, because the next entry simply covers a longer distance, but a partial fill breaks the method, since the tank is no longer full at both ends of an interval. The vehicle's trip computer is convenient, but checking it against your own log shows whether it reads high or low.
What to do with the result
Compare your real figure with the EPA estimates published at fueleconomy.gov. Differences of a few miles per gallon are common because driving style, speed, weather and load matter. For planning a trip, use your own number in the fuel cost calculator. If you changed tire sizes, the tire size calculator explains why the odometer and speedometer may read differently, which also changes the distance in this log.
Assumptions and limits
- Every fill-up is to full, and the tank was full at the starting odometer reading.
- Distance for a fill-up is the odometer difference from the previous entry, so readings must increase.
- The combined figure is total distance divided by total fuel. The average of the individual fill-ups is shown separately and can differ slightly.
- Prices are optional but must be entered for all fill-ups or none. Cost is price times fuel for each fill-up, rounded to the cent.
- Odometer error, partial fills, fuel left in the tank and the energy content of different fuels are not modeled.
- MPG here means US gallons. Conversions use 1.609344 km per mile and 3.785411784 liters per gallon.
Frequently asked questions
How do I calculate MPG?
Divide the miles driven by the gallons of fuel used. If you drove 268 miles and the next fill-up took 9.8 gallons, that is 27.35 MPG. This tool does it for each fill-up and for all of them together.
Why is the combined MPG different from the average of my fill-ups?
The combined figure weights each tank by its distance, so a long tank counts more than a short one. The plain average treats every tank equally. Usually the two are close, and the combined figure is the better measure.
Why is the first fill-up not counted?
It refilled the tank for miles driven before your log started, so its fuel does not match the distance you recorded. The distance is measured from that fill-up onward.
How do I convert MPG to L/100 km?
Divide 235.215 by the MPG. For 27.3 MPG that gives 8.62 L/100 km. The calculator shows both, so you do not need to convert by hand, and the L/100 km figure gets smaller when economy improves.
What if I miss a fill-up?
Just skip it. The next entry will show a longer distance and more fuel. As long as each fill is to full, the combined economy stays correct, though the individual tank figure covers more than one stop.