How to calculate MPG

A simple formula to figure out your car's real fuel economy.

Quick MPG calculator

miles
gallons

The MPG formula

MPG = Miles driven ÷ Gallons used

That's it. Divide the number of miles you drove by the number of gallons it took to refill your tank. The result is your miles per gallon.

Step-by-step example

Let's say you want to check the fuel economy on your car:

  1. Fill your tank all the way up. Note your odometer reading or reset your trip meter to zero.
    Example: odometer reads 45,000 miles.
  2. Drive normally until you need gas again. Don't change your driving habits — you want a realistic number.
  3. Fill up again and record two things: your new odometer reading and how many gallons it took to fill the tank.
    Example: odometer reads 45,350 miles. You pumped 12.5 gallons.
  4. Subtract the old reading from the new one to get miles driven.
    45,350 − 45,000 = 350 miles
  5. Divide miles by gallons.
    350 ÷ 12.5 = 28.0 MPG

Why your MPG might differ from the EPA rating

The number on your car's window sticker comes from EPA lab tests under controlled conditions. In real life, several factors change your actual fuel economy:

  • 🌡️ Weather. Cold temperatures thicken engine oil and reduce tire pressure, dropping MPG by 10–20% in winter.
  • 🏙️ City vs highway. Stop-and-go city driving uses significantly more fuel than steady highway cruising.
  • 🚀 Driving style. Hard acceleration and speeding above 50 mph reduces fuel economy. Every 5 mph over 50 is roughly like paying an extra $0.20/gallon.
  • ⛰️ Terrain. Hilly or mountainous routes use more fuel than flat roads.
  • 🧳 Cargo and passengers. Extra weight means the engine works harder. Roof racks also increase wind resistance.

Converting MPG to other units

If you need your result in metric units, here are the conversion formulas:

L/100km = 235.215 ÷ MPG

km/L = MPG × 0.425144

Or use our MPG converter to do it instantly.

Tips for getting an accurate reading

  • ✓ Always fill up completely — both at the start and end of your measurement. Partial fills give inaccurate results.
  • ✓ Measure over at least one full tank. A single tank gives a decent estimate. Averaging over 2–3 tanks is more accurate.
  • ✓ Use the trip odometer. It's more convenient than tracking the main odometer and you won't forget the starting number.
  • ✓ Don't trust your car's MPG display blindly. Built-in fuel economy displays can be off by 5–10%. Doing the math yourself is more reliable.