How this works
This calculator converts a single (distance, fuel-used) pair into all three common fuel-economy metrics. Drop in your trip-meter reading and the litres or gallons added at the pump and you get an instant cross-check between MPG, L/100km and km/L.
Why log multiple tanks. A single tank-to-tank measurement contains noise from how full the tank was at the start vs. the end (brim-filling matters), the route mix (motorway vs. city), driving style, and weather. The on-board computer in modern cars is convenient but is consistently optimistic by 5–10% — it under-reports throttle-on injection during transient acceleration. Logging 3–5 full tanks and averaging gives a number you can actually trust to within ~1–2 mpg.
Use this number for two things. (1) Validating manufacturer claims — compare your real consumption to the EPA/WLTP figure to see how badly your driving conditions deviate. (2) Budgeting fuel cost on the Fuel Cost calculator using your true consumption rather than a sticker number.
The formula
distance and fuel-used must come from the same trip. Brim-fill at the start and end so the "fuel used" equals what you added at the pump on the second fill — not what the dial says.
Example calculation
- Drove 600 km between two brim fills, added 45 L at the second.
- L/100km = 45 × 100 / 600 = 7.5. km/L = 600/45 = 13.33. MPG (US) = 235.215 / 7.5 = 31.4.
Frequently asked questions
Why does my car's computer show better MPG than this?
On-board fuel computers calculate consumption from instantaneous injector pulses. They are typically 5–10% optimistic because (a) they round in the user's favour and (b) they don't account for fuel that vapourises in the tank, used by the cold-start system, or burned during stop/start cranking. Trust the brim-to-brim "miles divided by litres" figure for absolute numbers; trust the on-board computer for trend and comparison.
How many tanks do I need to log for a reliable average?
Three to five full tanks driven in your usual route mix gets within ~1–2 mpg of the long-run average. If you commute through a mix of motorway and city, alternate tanks rather than picking a representative one. Don't average a road-trip tank with a winter-commuting tank; they're different operating regimes.
Does cold weather really hurt fuel economy that much?
Yes — typically 10–20% worse below freezing. Causes: thicker oil, longer warm-up phase running rich, denser air, increased rolling resistance from cold tyres, more electric load (heated seats, defroster, headlights), and shorter trips that never reach efficient operating temperature. Hybrids suffer most because the engine has to run more often just to provide cabin heat. Plan winter fuel budgets ~15% above your summer average.