Calcady™
Home / Unit / Fuel Efficiency Calculator

Fuel Efficiency Calculator

Convert fuel efficiency between MPG, L/100km, and km/L — includes trip fuel cost calculation for road trips and commutes.

Trip Metrics

US Standard (MPG)
25.0Miles/Gal
European Standard
9.4
L / 100km
Kilometers Per Liter
10.6
KM / L
Email LinkText/SMSWhatsApp

What is Fuel Economy Standards & Unit Conversions?

How MPG, L/100km, and km/L represent the same quantity from three different measurement paradigms — and why the inverse relationship between these metrics means low-MPG cars hold enormous fuel savings potential.

Mathematical Foundation

MPG (US Standard)
MPG=distancemilesfuelgallonsMPG = \frac{distance_{miles}}{fuel_{gallons}}
distancemilesdistance_{miles}= Total distance traveled in statute miles
fuelgallonsfuel_{gallons}= Total fuel consumed in US liquid gallons (1 US gal = 3.785 L)
L/100km (European Standard)
L/100km=fuellitersdistancekm×100L/100km = \frac{fuel_{liters}}{distance_{km}} \times 100
fuellitersfuel_{liters}= Total fuel consumed in liters
distancekmdistance_{km}= Total distance traveled in kilometers

Laws & Principles

  • The Inverse Relationship: MPG and L/100km are mathematical reciprocals of each other, not simply scaled versions. This means improving from 10 to 20 MPG saves far more fuel than improving from 40 to 50 MPG — even though the MPG gain is the same (10). In L/100km terms: 10→20 MPG = 23.5→11.8 L/100km (saves 11.7L), but 40→50 MPG = 5.9→4.7 L/100km (saves only 1.2L).
  • US Gallon vs Imperial Gallon: The US gallon (3.785 L) is different from the British Imperial gallon (4.546 L). A car rated at 30 US-MPG would achieve ~36 Imperial MPG using the same actual fuel and distance. British and Australian fuel economy ratings are historically quoted in Imperial units, creating frequent transatlantic confusion.

Step-by-Step Example Walkthrough

" A vehicle travels 300 miles and consumes 12 gallons of fuel. "

  1. MPG: 300 miles ÷ 12 gallons = 25.0 MPG.
  2. Convert to liters: 12 gal × 3.785 = 45.42 liters.
  3. Convert to km: 300 miles × 1.60934 = 482.8 km.
  4. L/100km: (45.42 ÷ 482.8) × 100 = 9.41 L/100km.
  5. km/L: 482.8 km ÷ 45.42 L = 10.63 km/L.
Final Result: 25.0 MPG = 9.41 L/100km = 10.63 km/L

Quick Answer: How Do You Convert MPG to L/100km?

Divide 235.215 by the MPG value. A car rated at 30 MPG = 235.215 ÷ 30 = 7.84 L/100km. The conversion is inverse — higher MPG means lower L/100km. Note: US gallons (3.785L) and Imperial gallons (4.546L) are different — this calculator uses US gallons by default.

MPG ↔ L/100km Quick Reference

MPG (US) L/100km km/L Typical Vehicle
15 MPG15.76.4Full-size truck (towing)
25 MPG9.410.6Mid-size SUV
35 MPG6.714.9Compact sedan
50 MPG4.721.3Hybrid sedan
130 MPGe1.855.3Full electric (MPGe equivalent)

L/100km is used in Canada, Europe, and Australia. Lower is better (inverse of MPG where higher is better). US and Imperial gallons differ — 1 Imperial gallon = 1.2 US gallons.

Fuel Efficiency Tips

Do This

  • ✓Use real-world MPG, not EPA window sticker. EPA ratings are based on lab testing. Real-world efficiency is typically 10-20% lower. Track your actual fill-ups over several tanks to get your true MPG for accurate trip cost calculations.
  • ✓Remember that MPG improvements are non-linear. Going from 15 to 20 MPG saves more fuel per mile than going from 35 to 50 MPG. A truck improving from 15→20 MPG saves 1.67 gallons per 100 miles. A car improving from 35→50 MPG saves only 0.86 gallons per 100 miles.

Avoid This

  • ✗Don't mix US and Imperial gallons. A UK car rated at "40 MPG" (Imperial) is actually 33.3 MPG (US). Imperial gallons are 20% larger. If you're comparing US and UK vehicle specs, convert to L/100km first — it's the universal standard.
  • ✗Don't assume highway MPG for city driving. City efficiency can be 25-40% worse than highway due to stop-start driving, idling, and acceleration losses. Plan commute costs using your city rating, not the combined or highway number.

Frequently Asked Questions

Why is L/100km used instead of km/L?

L/100km measures fuel consumption (how much fuel for a fixed distance), which scales linearly — 10 L/100km uses exactly twice as much fuel as 5 L/100km. MPG and km/L measure fuel economy (how far on a fixed amount of fuel) which is non-linear. This makes L/100km better for comparing savings: dropping from 10 to 8 L/100km saves the same amount as dropping from 6 to 4.8 L/100km.

What is MPGe for electric vehicles?

MPGe (Miles Per Gallon equivalent) measures how far an electric vehicle travels on the energy equivalent of one gallon of gasoline (33.7 kWh). A Tesla Model 3 at 130 MPGe means it travels 130 miles on 33.7 kWh of electricity. To calculate actual cost, use your electricity rate instead: 33.7 kWh × $0.13/kWh = $4.38 per "gallon equivalent" vs $3.50 for actual gasoline — but the EV travels 4× further on that energy.

How do I calculate trip fuel cost?

Divide trip distance by your MPG, then multiply by fuel price. A 500-mile trip in a 25 MPG car at $3.50/gallon: 500 ÷ 25 = 20 gallons × $3.50 = $70.00 in fuel. For round trips, double it. Add 10-15% for city driving segments, detours, and AC use — a 500-mile highway trip with city stops and AC will use closer to 22 gallons.

Related Calculators