Vehicle Fuel Economy Guide: MPG vs. L/100km, Road Trip Budgeting & the MPG Illusion
After depreciation, fuel is the largest ongoing operating expense of owning a motor vehicle. Whether you are commuting daily, planning a cross-country road trip, or deciding whether the higher purchase price of a hybrid vehicle will pay for itself at the pump, accurate fuel efficiency math puts you in control of your transportation budget.
How to Measure True Tank-to-Tank Fuel Economy
Dashboard trip computers often overestimate fuel economy by 3% to 7%. To measure your vehicle’s exact real-world efficiency:
- Fill your gas tank until the pump’s automatic shut-off clicks, and reset your Trip A odometer to
0.0. - Drive normally until your next fill-up.
- Refuel at the pump until it clicks off, and record both the miles (or kilometers) driven and the exact gallons (or liters) pumped from the receipt.
- Enter both numbers into our Fuel Efficiency Calculator.
The "MPG Illusion": Why L/100km Is Superior for Cost Comparisons
Researchers at Duke University’s Fuqua School of Business coined the term "The MPG Illusion" to describe how human intuition misjudges fuel savings when efficiency is expressed as Miles per Gallon. Suppose you drive 15,000 miles per year with gas at $3.60 per gallon. Which upgrade saves more fuel and money?
| Vehicle Upgrade Scenario | Annual Gallons Used (15,000 mi) | Annual Fuel Saved | Annual Cash Saved ($3.60/gal) |
|---|---|---|---|
| Scenario A: Trading a 15 MPG truck for a 20 MPG SUV (+5 MPG) | 1,000 gal → 750 gal | 250 gallons saved | $900.00 / year |
| Scenario B: Trading a 35 MPG sedan for a 50 MPG hybrid (+15 MPG) | 429 gal → 300 gal | 129 gallons saved | $464.40 / year |
Even though Scenario B increases MPG by three times as much (+15 MPG vs. +5 MPG), Scenario A saves nearly twice as much fuel and money! That is because fuel consumed is inversely proportional to MPG. When expressed in Liters per 100 km (L/100km) using the formula L/100km = 235.215 ÷ MPG, Scenario A drops from 15.7 to 11.8 L/100km (-3.9 L/100km), whereas Scenario B drops from 6.7 to 4.7 L/100km (-2.0 L/100km), making the true savings immediately obvious.
Planning Road Trip Travel Time & Financing
Combine our Fuel Efficiency Calculator with our Speed Calculator to estimate highway arrival times and our Auto Loan Calculator to compare monthly vehicle financing costs.