Before you book the road trip, know what it will actually cost to get there. The math is one short formula.
Fuel is one of the few travel costs you can calculate almost exactly before you leave. A 600-mile drive in a 30-MPG sedan at $3.50/gallon costs $70. The same drive in a 15-MPG SUV costs $140. The same drive in an EV charging at home costs about $25. This guide walks through the math, the global unit zoo, and the tricks that genuinely improve fuel economy.
Imperial (US, UK): Cost = (Distance in miles ÷ MPG) × Price per gallon
Metric (rest of world): Cost = (Distance in km × L/100km ÷ 100) × Price per litre
Drive 850 miles in a car rated 32 MPG. Fuel at $3.65/gal.
Drive 1,200 km in a car rated 6.5 L/100km. Fuel at €1.85/L.
Two systems measure the same thing inversely. MPG is miles per gallon — higher is better. L/100km is litres per 100 kilometres — lower is better.
Conversion: MPG (US) = 235.2 ÷ (L/100km). So 6 L/100km = 39.2 MPG.
Quick reference:
Note: UK gallons are 4.546 L; US gallons are 3.785 L. A British 50 MPG is roughly a US 42 MPG.
1. Slow down. Above 80 km/h, every 10 km/h costs roughly 10% more fuel. 2. Inflate tyres correctly. Under-inflation can waste 3% of fuel. 3. Lose dead weight. 50 kg of unused stuff in the boot ≈ 1–2% extra fuel. 4. Remove roof racks/boxes when not in use. Aerodynamic drag is huge. 5. Anticipate traffic. Smooth driving > brake/accelerate cycles. 6. Use cruise control on highways. 7. Avoid idling. 60+ seconds of idling burns more fuel than restarting. 8. Combine errands so the engine stays warm. 9. Use the right octane — premium in a regular-spec engine is wasted money. 10. Service on schedule — clogged air filter or dirty injectors silently waste fuel.
Modern EVs use roughly 17 kWh per 100 km. At home electricity prices:
Compare to a 30 MPG gas car at $3.50/gallon: $7.32 per 100 km. EVs are typically 50–70% cheaper to fuel per km, though purchase price and battery degradation must enter the total-cost calculation.
Does the AC really hurt MPG? At highway speed, AC costs 5–10% in fuel; at city speed, similar. Open windows above 80 km/h cost almost as much from drag.
Why do real-world MPG differ from rated figures? Manufacturer tests use ideal conditions. Real-world is typically 10–25% worse, especially in cold weather.
Diesel vs petrol? Diesel produces ~20% better fuel economy and was historically cheaper. Modern emissions rules narrowed the gap.
Use our Fuel Cost Calculator for instant trip estimates in any currency and unit system.