Converting Fuel Economy and Speed Between Measurement Systems
Why MPG and L/100km run in opposite directions, and the speed conversion formula worked through with real numbers.
Fuel economy and speed conversions both involve two measurement systems that most drivers eventually have to translate between — whether comparing a car's specs across markets or reading a foreign speedometer.
Fuel economy: an inverted relationship
L/100km = 235.215 / MPG. A car rated at 30 MPG converts to 235.215/30 ≈ 7.84 L/100km. Notice the relationship runs in reverse — a higher MPG corresponds to a lower L/100km, since MPG measures distance per unit of fuel (bigger is better) while L/100km measures fuel per unit of distance (smaller is better). There's no simple multiplication here; it's an inverse relationship, which is why a linear conversion factor alone (like meters-to-feet) wouldn't work for this pair.
Speed: a straightforward multiplication
km/h to mph: multiply by 0.621371. A car traveling 100 km/h converts to 100 × 0.621371 ≈ 62.14 mph. Unlike fuel economy, speed conversion is a simple linear relationship, since both units measure the same thing (distance per time) in the same direction.
Why fuel economy needs the inverse formula but speed doesn't
Speed conversion just rescales the same type of ratio (distance/time in one unit, converted to distance/time in another). Fuel economy conversion, though, flips from a "distance per fuel" ratio (MPG) to a "fuel per distance" ratio (L/100km) — genuinely inverting the ratio's numerator and denominator, not just rescaling it, which is exactly why the conversion formula involves division into a constant rather than simple multiplication.
A quick sanity check for fuel economy conversions
Because the relationship is inverse, doubling a car's MPG doesn't halve its L/100km value in a simple, linear way across the full range — but for cars within a similar efficiency range, roughly doubling MPG does roughly halve L/100km, since the inverse relationship behaves close to linearly over a limited range even though it isn't linear in general.
Common mistakes to avoid
- Applying a simple multiplication factor to fuel economy conversions, as if it worked the same way as speed or length conversions
- Confusing US MPG with UK/Imperial MPG, which use different gallon sizes (US gallon ≈ 3.785L, Imperial gallon ≈ 4.546L) and therefore produce different MPG figures for the same actual fuel efficiency
- Forgetting that both conversions assume consistent driving conditions — real-world fuel economy varies with terrain, speed, and load in ways no unit conversion can capture
Convert your own numbers with the fuel economy converter and speed converter.