Core answer: Fuel consumption = fuel used ÷ distance × 100 (L/100km). Fill the tank, zero the trip meter, drive normally, refill: L/100km = liters added ÷ km driven × 100. Example: 42 L over 520 km = 8.1 L/100km. At ¥8/L that's ¥0.65/km. EVs flip the unit: kWh/100km — a 15 kWh/100km EV at ¥0.55/kWh home rate costs ¥0.08/km, one eighth of the gasoline car.
The full cost picture
| Vehicle | Consumption | Energy price | Cost per 100 km |
|---|---|---|---|
| Compact gasoline | 6.5 L | ¥8/L | ¥52 |
| SUV gasoline | 9.5 L | ¥8/L | ¥76 |
| Hybrid (HEV) | 4.5 L | ¥8/L | ¥36 |
| EV (home charge) | 15 kWh | ¥0.55/kWh | ¥8.3 |
| EV (public fast) | 15 kWh | ¥1.5/kWh | ¥22.5 |
Why your real number differs from the brochure
- NEDC/CLTC ratings are lab cycles; real-world runs 15–30% higher (CLTC EVs especially optimistic on range).
- Cold weather: gasoline engines +10–20% (rich warm-up), EVs −20–40% range (battery chemistry + cabin heat).
- Speed: aero drag grows with v²; 120 km/h burns ~30% more per km than 90 km/h.
- Load and tires: +100 kg ≈ +3–5%; underinflated tires (+0.5 bar low) ≈ +2–4%.
Worked examples
Example 1 — The honest measurement. Fill to first click, reset trip, drive 486 km, refill 39.2 L: 39.2 ÷ 486 × 100 = 8.07 L/100km — your number, not the forum's.
Example 2 — Annual budget. 15,000 km/year at 8 L/100km and ¥8/L: 1,200 L = ¥9,600/year. A hybrid alternative at 4.5 L saves ¥4,200/year; the EV at home rates saves ~¥8,400/year on energy alone.
Example 3 — Highway vs city. Same car: city 10.5, highway 6.5 L/100km. A 50/50 driver averaging 8.5 who moves to a highway commute at 6.8 saves 255 L/year = ¥2,040.
Example 4 — The idling math. Idling burns ~0.8–1.5 L/h; 20 min/day of warm-up idling ≈ 0.4 L × 300 days = 120 L = ¥960/year — remote starters are comfort, not economy.
Common mistakes and myths
- Trusting the dash computer — optimistic by 5–10% typically; the pump-to-pump method is the only truth.
- Single-tank judgments — wind, load, and traffic swing one tank by ±15%; average 3+ tanks before conclusions.
- "Higher octane saves fuel" — 95 vs 92 changes economy ~0–2% in engines designed for 92; the ¥0.5/L premium buys knock resistance, not mileage.
- EV winter denial — a 500 km CLTC EV can deliver 280–320 km at −10 °C with heating; plan winter legs accordingly.
- MPG vs L/100km confusion — MPG (US) improves upward, L/100km improves downward; converting: L/100km = 235.2 ÷ MPG. A 35 MPG car = 6.7 L/100km.