Fuel Savings Calculator

Fuel economy: cost per km = price per liter / km per liter

Comparing fuels really comes down to cost per kilometer: cost_km = price_liter / consumption_kmL. Say gasoline runs R$ 6.00/L in a car doing 13 km/L, that's R$ 0.46/km; ethanol at R$ 4.00/L in the same car at 9 km/L lands at R$ 0.44/km, so ethanol takes it. For flex cars there's the 70% rule: ethanol pays off once its price drops below 70% of gasoline, because a liter of ethanol carries about 70% of gasoline's energy. A few habits help on the driving side. Cruise around 80–90 km/h, the band where rolling resistance and aerodynamic drag balance out lowest. Go easy on the throttle and the brakes. Keep tires inflated, since a 0.2 bar drop pushes consumption up roughly 3%. Use the A/C with some judgment (5–15% extra on the highway, though still cheaper than open windows once drag kicks in), and remember that each extra 100 kg of weight costs about 5% in consumption.

Applications

Planning a road trip and weighing gasoline against ethanol, choosing between Uber and your own car, ride-share drivers sizing up which vehicle to use, a monthly commuting budget, or sizing up a hybrid or electric car, where cost per km usually sits well below an ICE vehicle.

FAQ

Does A/C really increase consumption? It does, by 5 to 15% more on the highway. But once you're past 80 km/h, open windows create so much drag that the A/C turns out to be the cheaper choice.

Why is the optimal speed 80–90 km/h? Slower than that and the engine is stuck working inefficiently in lower gears. Faster, and aerodynamic drag grows with the square of speed until it takes over.

How accurate is the 70% rule? For most flex cars it's a solid approximation, though real efficiency shifts from engine to engine. If you want the exact answer, run a full tank of each fuel and compare.