How is trip fuel cost calculated?
Divide the distance by your fuel economy to get the volume needed, then multiply by the price per unit. A 300-mile trip at 30 mpg needs 10 gallons; at 4 per gallon that is 40. In metric terms, multiply distance by litres per 100 km, divide by 100, then multiply by price per litre. The arithmetic is simple; the accuracy depends entirely on using a realistic economy figure.
Why is my real fuel economy worse than the official figure?
Official ratings come from standardised test cycles that few people replicate. Real-world consumption is worse because of cold starts, short journeys where the engine never reaches efficient temperature, air conditioning, roof boxes and roof bars, under-inflated tyres, extra weight, and above all speed — aerodynamic drag rises with the square of velocity, so 80 mph can use 20–25% more fuel than 65 mph over the same distance. Expect 10–20% worse than the sticker.
Should I use mpg or litres per 100 km?
They express the same thing inversely, but litres per 100 km is easier to reason about because it scales linearly — halving it halves your fuel use. With mpg, improvements at the low end matter far more than at the high end: going from 15 to 20 mpg saves more fuel over the same distance than going from 40 to 50. That non-linearity misleads people comparing efficient cars.
How do I compare an electric vehicle's running cost?
Work in cost per distance rather than per unit of energy. Divide the price per kWh by the vehicle's efficiency in miles or km per kWh. A car doing 3.5 miles per kWh charged at 0.30 per kWh costs about 0.086 per mile, against roughly 0.13 for a 30 mpg petrol car at 4 per gallon. Public rapid charging can cost several times home charging, so which tariff you assume changes the answer substantially.
Is it worth driving further to a cheaper station?
Usually not, and the arithmetic is quick. A 10-mile detour in a 30 mpg car burns a third of a gallon, so on a 12-gallon fill you need a saving of about 3 cents per gallon just to break even on fuel — before your time. Detours pay off only for large tanks, large price differences, or when the station is on your route anyway.
What actually reduces fuel costs the most?
In rough order of impact: driving more slowly on motorways, smoother acceleration and braking, keeping tyres correctly inflated, removing roof racks and unnecessary weight, combining short trips into one longer one so the engine warms once, and keeping the vehicle serviced. Together these routinely deliver 15–25% — more than most people save by shopping around on price.