Guide
Fuel Cost and Fuel Efficiency: Formulas, Conversions and Trip Math
The cost of a drive is distance, efficiency and price multiplied together. This guide gives the formulas in litres-and-kilometres and gallons-and-miles, works through a road trip and a yearly comparison, shows how to compare with charging an electric vehicle, and explains why real-world consumption varies. All prices are examples you should replace with your own.
By MyCalcsWorld editorial · Last reviewed 26 Oct 2026 · 4 min read
The two measurement systems
Fuel efficiency is quoted in two opposite ways. Distance per volume (miles per gallon, kilometres per litre) rises as efficiency improves. Volume per distance (litres per 100 km) falls as efficiency improves. Use whichever your car and fuel prices use, and convert carefully; see the unit-conversion guide for the formulas and the US versus imperial gallon issue.
Formulas for trip cost
With consumption in litres per 100 km:
fuel used (L) = distance (km) ÷ 100 × consumption (L/100 km)
cost = fuel used × price per litre
With efficiency in miles per gallon:
fuel used (gal) = distance (mi) ÷ mpg
cost = fuel used × price per gallon
Cost per distance is then cost ÷ distance, which is the number to use when sharing costs or claiming mileage.
Worked example: a 500 km trip
A car uses 7.5 L/100 km. Fuel costs $1.60 a litre (an example price).
- Fuel used: 500 ÷ 100 × 7.5 = 37.5 L.
- Cost: 37.5 × $1.60 = $60.00.
- Cost per kilometre: $60 ÷ 500 = $0.12.
If four people share the trip, each pays $15.00 for fuel alone. Tolls, parking and wear on the car are extra.
Worked example in miles and gallons
A 300-mile trip at 28 US mpg with fuel at $3.80 per US gallon uses 300 ÷ 28 ≈ 10.71 gallons, costing about $40.71, or about $0.136 per mile.
What a small efficiency difference is worth
Suppose you drive 15,000 km a year with fuel at $1.60 a litre. At 8.0 L/100 km the annual fuel cost is 150 × 8.0 × $1.60 = $1,920. At 6.5 L/100 km it is 150 × 6.5 × $1.60 = $1,560. The difference of 1.5 L/100 km is worth $360 a year. This kind of calculation helps compare vehicles or judge whether a driving-style change matters.
Comparing with an electric vehicle
Electric consumption is given in kilowatt-hours per 100 km (or miles per kWh). The cost per 100 km is consumption × electricity price. For an illustrative 18 kWh/100 km and $0.15 per kWh, the energy cost is 18 × $0.15 = $2.70 per 100 km, compared with 7.5 × $1.60 = $12.00 per 100 km for the petrol car above. Charging losses, public charging prices, which are often higher than home rates, and the vehicle’s purchase price all affect the true comparison, so treat this as a framework and use your own tariffs.
Why real consumption differs
- Speed. Aerodynamic drag grows with the square of speed, so consumption typically rises at highway speeds.
- Driving style. Hard acceleration and braking use more fuel.
- Load and equipment. Extra weight, roof racks and air conditioning increase use.
- Conditions. Cold weather, hills, wind and congested traffic change consumption.
- Maintenance. Under-inflated tyres raise rolling resistance.
Official ratings come from standardised tests and often differ from real use. A good habit is to measure your own: fill the tank, note the odometer, drive, refill, and divide the fuel added by the distance driven.
Sharing costs and claiming mileage
When sharing a trip, split fuel plus tolls and parking, and agree whether to include a contribution to wear. For mileage allowances, use the rate set by your employer or tax authority rather than your own fuel cost, since allowances are meant to cover more than fuel.
Budgeting a monthly commute
Suppose you drive 40 km a day to work, 22 days a month. That is 880 km, which at 7.5 L/100 km uses 66 litres. At $1.60 a litre, the month’s fuel costs 66 × $1.60 = $105.60. A price rise of $0.10 a litre adds only $6.60 a month for this driver, which helps put short-term price swings in perspective. Compare that with the cost of parking, insurance, maintenance and depreciation, which are often larger than fuel.
What actually reduces consumption
- Maintain steady speeds and anticipate traffic so you brake less.
- Keep tyres inflated to the pressure in the vehicle’s documentation.
- Remove roof racks and unnecessary load when you do not need them.
- Combine short trips, since a cold engine uses more fuel per kilometre.
- Plan routes that avoid heavy congestion and long idling.
The size of each effect depends on the vehicle and conditions, so track your own consumption before and after a change instead of relying on generic percentages.
Common mistakes
- Mixing US and imperial gallons.
- Using a manufacturer rating when your measured consumption is higher.
- Forgetting round-trip distance.
- Ignoring tolls and parking in a trip budget.
- Comparing mpg and L/100 km figures directly without converting.
Using the calculators
The Fuel Cost Calculator takes distance, efficiency and price. The Fuel Economy Converter switches between mpg and L/100 km, and Fuel vs EV Cost sets the two side by side. Use prices from your local station and tariff.
FAQ
Frequently asked questions
- How do I calculate fuel cost for a trip?
- Divide distance by efficiency (or multiply by consumption per distance) to get fuel used, then multiply by the price per unit.
- How do I convert mpg to L/100 km?
- Divide 235.215 by US mpg, or 282.481 by imperial mpg.
- Is a higher mpg or a lower L/100 km better?
- Higher mpg is better; lower L/100 km is better. They are inverse scales.
- Why is my real mileage worse than the rating?
- Ratings come from standardised tests. Speed, load, traffic, weather and driving style can raise real consumption.
- How do I compare petrol and electric cost per distance?
- Multiply consumption by price for each: litres per 100 km times price per litre, and kWh per 100 km times price per kWh. Include charging losses and purchase price for a full comparison.