FuelTripCost guide
What Is MPG and How Is It Calculated?
By FuelTripCost · Published 27 September 2026 · Updated 30 September 2026
Quick answer
MPG means miles traveled per gallon of fuel consumed. In the US, it normally means miles per US gallon. Higher MPG indicates less fuel used for the same distance.
Miles driven ÷ US gallons used = US MPG.
What you need
- Miles driven over a defined interval.
- US gallons consumed over that same interval.
Step-by-step method
- Begin with a full tank and record the odometer.
- Drive normally.
- Refill to a comparable level and record gallons added.
- Subtract odometer readings or use the trip meter.
- Divide miles by US gallons and repeat over several tanks.
Worked examples
Core example
If you travel 330 miles and add 11 US gallons at the next full fill-up, your measured economy is 30 US MPG for that interval.
30 MPG interval
330 miles ÷ 11 US gallons = 30 US MPG.
Different interval, same economy
150 miles ÷ 5 gallons also equals 30 MPG.
Combining intervals
If two intervals total 600 miles and 21 gallons, combined economy is 600 ÷ 21 = 28.57 MPG. Do not average the two MPG values unless the intervals use equal fuel.
Tank-to-tank measurement
Start and end at comparable full levels so fuel added at the ending fill approximates fuel consumed during the recorded miles. A partial fill alone cannot provide a complete interval because the ending fuel level is unknown.
You do not need to drive until nearly empty. A longer valid interval simply reduces the relative influence of small fill-level differences.
Combining several MPG intervals
Add all valid interval miles and all corresponding US gallons, then divide the totals. Do not take a simple average of MPG readings when the intervals used different amounts of fuel.
Keep notes about traffic, weather, towing, load, and route. A multi-tank result describes those conditions and may differ from a dashboard display or official rating without proving that either is defective.
Real-world considerations
- Start and end with a full tank for a better fill-up estimate.
- Record the miles from a trip meter rather than guessing.
- One tank can be unusual; several intervals give a more representative planning figure.
Common mistakes
- Using gallons purchased at a partial fill-up.
- Averaging MPG values without weighting miles or gallons.
- Assuming Imperial MPG is numerically identical.
How to make the estimate more useful
- Track totals across several full-tank intervals.
- Record unusual towing, idling, or weather so the number has context.
Limits of this calculation
A fill-up estimate is affected by pump shutoff, fill level, odometer accuracy, and driving conditions. It is not a laboratory measurement.
Questions
Is UK MPG the same number?
No. An Imperial gallon is larger than a US gallon, so the numerical MPG is different for the same fuel use.
Is higher MPG better?
Yes. Higher MPG means more distance from each gallon.
Can MPG be converted to L/100 km?
Yes. For US MPG, L/100 km is approximately 235.214583 divided by MPG.
Use the calculation
Try the Gas Mileage Calculator or review the calculation methodology and unit constants.