Gas Mileage Calculator
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Fill the tank, write down the odometer, drive, then fill up again: MPG = miles driven ÷ gallons used. Driving 320 miles on 11.4 gallons is 28.1 MPG, which is 8.38 liters per 100 km.
Odometer now minus odometer at the last fill-up.
Gallons or liters it took to fill the tank again.
| Fuel economy | |
| Miles per gallon (US) | 28.07 |
| Liters per 100 km | 8.38 |
| Kilometers per liter | 11.93 |
| Miles per gallon (UK) | 33.71 |
| Cost | |
| Fuel cost for this distance | $37.05 |
| Cost per mile | $0.116 |
| Yearly fuel cost (12,000 miles) | $1,389 |
Show steps
- MPG = miles ÷ gallons = 320 ÷ 11.4 = 28.07.
- L/100 km = 235.215 ÷ 28.07 = 8.38.
How to measure gas mileage
- Fill the tank until the pump clicks off and write down the odometer reading (or reset the trip meter).
- Drive as usual until you need gas again.
- Fill up again the same way and note the gallons on the pump receipt.
- Enter the miles driven and the gallons, then press Calculate.
Averaging over a few tanks gives a steadier number than one fill-up.
Formulas
- MPG = miles ÷ gallons
- L/100 km = liters ÷ km × 100 = 235.215 ÷ MPG
- km/L = km ÷ liters = MPG × 0.4251
- Cost per mile = fuel price ÷ MPG
MPG measures distance per fuel, so higher is better. L/100 km measures fuel per distance, so lower is better.
MPG and fuel cost
| MPG | L/100 km | Gallons per 12,000 miles | Cost at $3.25/gal |
|---|---|---|---|
| 15 | 15.7 | 800 | $2,600 |
| 20 | 11.8 | 600 | $1,950 |
| 25 | 9.4 | 480 | $1,560 |
| 30 | 7.8 | 400 | $1,300 |
| 40 | 5.9 | 300 | $975 |
| 50 | 4.7 | 240 | $780 |
Going from 15 to 20 MPG saves 200 gallons a year, more than going from 30 to 50 MPG saves (160 gallons). That is why upgrading the least efficient car often saves the most.
Gas mileage practice questions
Test yourself with 6 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.
Quiz complete
Frequently asked questions
How do I calculate miles per gallon?
Divide the miles you drove since your last full tank by the gallons it took to refill. 280 miles ÷ 10 gallons = 28 MPG.
How do I convert MPG to L/100 km?
Divide 235.215 by the MPG (US gallons). 25 MPG is 9.41 L/100 km.
Why is my real MPG lower than the window sticker?
Short trips, cold weather, idling, fast driving, roof racks, low tire pressure and heavy loads all lower real-world mileage compared with standard test cycles.
Are US and UK MPG the same?
No. A UK (imperial) gallon is about 1.2 US gallons, so the same car shows a higher MPG in UK figures. 30 US MPG is about 36 UK MPG.
Sources
- U.S. Department of Energy: fueleconomy.gov, driving more efficiently
- NIST: SI units and US customary conversions
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