Gas Mileage Calculator
Calculate Fuel Economy
Result
| Gas Mileage (US) | — |
|---|---|
| Gas Mileage (UK) | — |
| Economy (km/L) | — |
| Economy (L/100km) | — |
How to Use This Gas Mileage Calculator
Dashboard MPG readouts drift over time, and I have seen drivers swear their car still hits sticker economy while the fill-up receipts tell a different story. This calculator turns two numbers you already have—distance driven and fuel pumped—into every major economy format in one shot.
- Enter Distance Driven. Use trip odometer miles or kilometers for one tank cycle, or the distance between two fill-ups.
- Enter Fuel Used. Record gallons (US or UK) or liters from the pump receipt at the fill-up that completes the cycle.
- Press Calculate. Results show US MPG, UK MPG, km/L, and L/100 km simultaneously.
Feed the US MPG result into the Fuel Cost Calculator for trip budgeting, or use the Mileage Calculator when you prefer main odometer start/end readings plus cost per mile.
Gas Mileage Formulas and Practical Applications
Fuel economy is distance divided by fuel volume—how far you went on how much you bought. Think of it like miles-per-dollar at the pump, except we measure distance per gallon or liter instead of cash.
Fuel Economy = Distance Driven ÷ Fuel Used
For L/100 km, invert the relationship: liters per 100 km equals fuel used divided by distance in km, times 100. Lower L/100 km means better economy; higher MPG means the same thing on the imperial side.
The fill-up method
Fill until the nozzle clicks, reset the trip meter, drive your normal mix of city and highway, then fill again. If the trip shows 300 miles and the pump dispenses 12 US gallons, economy is 25 MPG. Repeat across seasons to catch winter idle time or summer A/C load.
Standard Units and Conversion Tables
Different regions quote economy differently. This tool outputs all four common labels so you can compare to a European review, a US Monroney sticker, or your own historical log.
- US MPG: Miles per US gallon. Higher is better.
- UK MPG: Miles per imperial gallon—numerically higher than US MPG for identical consumption.
- km/L: Common in Japan; higher is better.
- L/100 km: EU/Canada standard; lower is better.
Cross-reference example
25 US MPG ≈ 10.6 km/L ≈ 9.4 L/100 km ≈ 30 UK MPG. Use measured fill-up data rather than converting sticker values when tracking wear or tuning changes.
Frequently Asked Questions
Why is my calculated gas mileage different from the sticker MPG?
Window-sticker ratings come from controlled lab tests. Real driving—traffic, hills, cold starts, A/C, and cargo—almost always shifts your actual economy up or down from that number.
Is it better to measure in MPG or L/100km?
Use whichever your region prefers. MPG shows distance per gallon; L/100km shows liters needed per fixed distance, which some drivers find easier for trip fuel planning. This calculator outputs both.
What is the fill-up method?
Fill the tank until the pump clicks off, reset the trip odometer, drive until the next fill-up, then enter distance driven and fuel pumped. That gives a real-world economy figure for that driving segment.
Why does the calculator show both US and UK MPG?
US and UK gallons differ in volume. The same distance and fuel volume produce different MPG values depending on which gallon definition you use, so both are listed for international comparison.
How much driving do I need for an accurate reading?
A full tank cycle on your normal routes works well—often 200 to 400 miles. Very short trips with lots of cold starts can skew results low because engines use extra fuel while warming up.