Why does my real mpg differ from the window sticker?

EPA window-sticker ratings are measured under standardized lab conditions; real-world mpg varies with driving style, terrain, HVAC use, tire pressure, and vehicle load.

Should I average multiple fill-ups?

Yes, if you want a representative figure — a single tank can be skewed by one unusually short or highway-heavy trip.

What's the difference between L/100km and km/L?

L/100km (used across most of Europe and elsewhere) is fuel used per fixed distance — lower is better. km/L (used in parts of Asia) is distance per fixed fuel amount — higher is better. Both describe the same underlying efficiency.

All tools

Math & Everyday

MPG Calculator

Real fuel economy from your own fill-up, in mpg, L/100km, or km/L.

🔒 Runs entirely in your browser — nothing here is ever uploaded

Remove ads →

About the MPG Calculator

Computes real-world fuel economy (mpg, L/100km, or km/L) from your own fill-up — distance driven divided by fuel used — rather than a manufacturer-published rating. Supports averaging across multiple fill-ups.

100% Free Runs in Your Browser No Sign-Up Required

How to use it

Enter the distance you drove and the fuel used to cover it.
Add more fill-ups for a multi-tank average, if you want a more realistic real-world figure.
Read your fuel economy in mpg, L/100km, and km/L.

Formula

mpg = total miles driven ÷ total gallons used. L/100km = (total liters ÷ total km) × 100. km/L = total km ÷ total liters. 1 mile = 1.609344 km, 1 US gallon = 3.785412 liters.

Worked example

312 miles on 9.75 gallons: 312 ÷ 9.75 = 32.0 mpg (7.35 L/100km, 13.6 km/L).

Recommendations

• A single tank can be skewed by an unusually short trip, cold starts, or highway-heavy driving — add 2-3 fill-ups for an average that's more representative of your typical use.
• This measures your actual real-world economy, which commonly runs below a vehicle's window-sticker combined rating, especially with AC/heat use or a loaded vehicle.
• Already know your mpg and want total trip cost instead? Use the Gas Trip Cost Calculator or Car Total Cost of Ownership Calculator, which take mpg as an input.

Frequently asked questions

Those take your mpg as an input to estimate cost. This one computes mpg itself from a real distance-and-fuel fill-up — use this first if you don't already know your vehicle's real mpg.