Fuel Economy Converter

Why L/100km and kWh/100km need different math than MPG or km/L

Most units on this site convert by multiplying: 1 inch is always 2.54 cm, whatever number you start with. Fuel economy has a second complication on top of that: some of these units get bigger for a more efficient vehicle (MPG, km/L, mi/kWh, km/kWh — more distance per unit of fuel is better), while others get smaller (L/100km, kWh/100km — less fuel to cover the same distance is better). Converting between the two families isn't a simple scale — it's a genuine reciprocal: km/L isn't L/100km multiplied by something, it's 100 divided by it.

That's also why doubling one number doesn't halve the other in a way you'd expect: a car that improves from 20 to 40 MPG cuts its fuel use in half (from 5 L/100km to 2.5 L/100km, roughly) — but a car that improves from 40 to 60 MPG only cuts fuel use by about a third, even though the MPG gain looks similar on paper. This is a well-documented, genuine source of confusion in fuel economy ratings, precisely because of this reciprocal relationship.

Comparing a gas car to an EV: what MPGe does and doesn't tell you

There's no gallon in an EV, so a direct MPG comparison isn't possible — which is exactly why the US Environmental Protection Agency created MPGe (miles per gallon of gasoline-equivalent) in 2010. The EPA defines one gallon of gasoline as containing the same energy as 33.7 kWh of electricity (based on gasoline's energy content of about 115,000 BTU). An EV that travels 100 miles on 33.7 kWh is rated at 100 MPGe — put simply, MPGe answers "if this EV were burning gasoline instead, what MPG would give the same energy use?" This converter uses that same official 33.7 kWh figure for every EV-related unit here, so MPG (US) and MPGe are, mathematically, the exact same scale by design — that's the whole point of the metric.

Worth knowing before you treat MPGe as the final word: it's an energy-content comparison, not a full efficiency or environmental comparison. It doesn't account for how the electricity was generated (a power plant converting fuel to electricity has its own losses, just as an engine converting gasoline to motion does) or for regional differences in electricity and gasoline prices — the EPA itself notes MPGe isn't a cost-equivalence figure. What MPGe reliably tells you is real, though: EVs consistently score far higher than gas cars in MPGe (2024 model-year EVs ranged roughly 53–140 MPGe, versus 9–57 MPG for gas and hybrid vehicles), because electric motors are fundamentally more efficient at turning stored energy into motion than combustion engines are — that part of the comparison holds up regardless of the caveats above.

A few other units exist in this space that aren't included as separate entries here, since they're simple rescalings of units already on this page: Wh/mi and Wh/km (watt-hours per mile/km, a finer-grained version of mi/kWh and km/kWh often quoted in EV enthusiast and press coverage), and kWh/100mi (the US-mile mirror of kWh/100km). Converting to any of these is just a matter of moving a decimal point or swapping the distance unit once you have the figures above.