GuidesFuel Economy Units Basics
Fuel Economy Units Basics
Understand L/100 km vs mpg thinking through speed, distance, and volume—convert trip length and pace correctly before you compare car efficiency figures.
By the Units Converters editorial team · Voyager Tecnologias LTDA
Economy combines volume and distance
Fuel economy answers “how much fuel per distance” (L/100 km) or “how much distance per fuel” (mpg). Those are inverse styles: lower L/100 km is better; higher mpg is better. You cannot convert them by treating the number like a simple length.
This site’s converters handle the building blocks—kilometres vs miles, litres vs other volumes, km/h vs mph for trip pace. Use them to sanitize inputs, then apply a proper economy formula or a dedicated economy calculator when comparing L/100 km to mpg.
US mpg usually means US gallons; UK historical mpg used imperial gallons. Always note which gallon. Metric countries lean on L/100 km or km/L.
Trip math before economy math
Record fuel volume added and distance driven in consistent units. Convert miles → kilometres (× 1.609344) if your odometer and pump disagree across a border rental. Convert gallons → litres if the receipt and the car computer disagree.
L/100 km ≈ (litres used × 100) / kilometres driven. km/L ≈ kilometres / litres. Those formulas only work after unit cleanup.
Partial fill-ups and forgotten top-ups ruin the dataset faster than unit mistakes. Start economy tests from full-to-full when you can.
Speed, distance, and why pace shows up
Highway efficiency depends on speed. Comparing a US road trip logged in mph with a European test cycle talked about in km/h needs speed conversion so you compare similar driving, not just similar labels.
Average speed = distance / time. Convert distance and speed into one system before judging whether a tank was “mostly highway.” A 70 mph cruise is about 113 km/h—not a European “70” zone.
Idle time burns fuel with zero distance, wrecking L/100 km. Duration units (hours of idle) explain the hit; speed converters do not erase it.
mpg and L/100 km conceptual bridge
Rough intuition: more miles per gallon means fewer litres per 100 km. Exact conversion needs both the mile–kilometre factor and the gallon–litre factor for the correct gallon. Do not invert the number alone (mpg → 1/mpg) and call it L/100 km.
Example directionality: about 8 L/100 km is roughly 29–30 US mpg (order of magnitude check). Always recompute with exact factors for claims or regulations.
Car stickers from different markets may already be converted—or may use different test cycles entirely. Unit conversion cannot fix a cycle mismatch (WLTP vs older cycles).
Volume units at the pump
Litres dominate most of the world. US pumps use US gallons (≈ 3.785 L). Confusing imperial gallons (≈ 4.546 L) with US gallons skews mpg badly.
Cubic metres appear in industrial fuel gas, not petrol stations (1 m³ = 1,000 L). Convert bulk deliveries carefully before computing fleet economy.
Trip computers may let you switch L/100 km ↔ mpg. After switching, reset averages; old samples mixed under another unit display still confuse humans even if the computer converted correctly.
Liste de contrôle pratique
Pick economy style (L/100 km or mpg). Convert distance and fuel volume into the units that formula needs. Convert highway pace with km/h ↔ mph when comparing trips across countries. Only then compute or convert the economy figure with the correct gallon definition.
Label every log line: distance unit, volume unit, economy style. Dual-display dashboards are fine; dual-meaning spreadsheets are not.
Use kilometre ↔ mile and km/h ↔ mph pages to clean trip data, and litre ↔ cubic metre when bulk fuel deliveries show up in m³. Apply a dedicated L/100 km ↔ mpg formula when you need the economy number itself.
Convertisseurs associés
Questions fréquentes
Is higher mpg always better than lower L/100 km?
They run in opposite directions: higher mpg is better; lower L/100 km is better. Convert with proper factors before comparing a US mpg sticker to a metric L/100 km sticker.
Can I convert mpg by flipping the number?
No. You need both distance (mile↔km) and volume (gallon↔litre) factors, and you must know whether the gallon is US or imperial.
Why convert speed when talking about fuel economy?
Economy depends strongly on driving speed. Converting km/h ↔ mph lets you compare trips or test conditions fairly before you compare L/100 km or mpg results.