How to Calculate Fuel Cost — Trip Planning & Efficiency Guide

The Science of Fuel Cost Calculation

Fuel cost estimation is a straightforward application of unit conversion and proportional reasoning. The key is understanding your vehicle’s fuel economy—how efficiently it converts fuel into distance—and combining it with current fuel prices.

Core Formulas

Metric System (Litres per 100 km):
Fuel Needed (L) = (Distance (km) × Consumption (L/100 km)) ÷ 100
Total Cost = Fuel Needed × Price per Litre

Imperial System (Miles per Gallon):
Fuel Needed (gal) = Distance (mi) ÷ MPG
Total Cost = Fuel Needed × Price per Gallon

Cost per Unit Distance:
Cost per km = (Consumption (L/100 km) × Price per L) ÷ 100
Cost per mile = Price per gal ÷ MPG

Example (Imperial):

• Trip: 300 miles
• Car: 25 MPG
• Fuel: £1.80/gallon
• Fuel needed: 300 ÷ 25 = 12 gal
• Total cost: 12 × £1.80 = **£21.60**

Understanding Fuel Economy Metrics

• MPG (Miles per Gallon): Common in the US and UK. Higher is better.
• L/100 km (Litres per 100 km): Standard in Europe and Canada. Lower is better.
• Conversion: L/100 km = 235.215 ÷ MPG (for US gallons)
  Example: 30 MPG ≈ 7.84 L/100 km

Real-World Factors That Affect Accuracy

Theoretical fuel economy rarely matches real-world performance due to:

• Driving style: Aggressive acceleration/braking reduces efficiency by 15–30%.
• Speed: Fuel economy drops sharply above 55–60 mph (88–96 km/h) due to aerodynamic drag.
• Load and accessories: Roof racks, heavy cargo, and AC use increase consumption.
• Terrain: Hills and mountains significantly impact fuel use.
• Vehicle condition: Under-inflated tires, poor maintenance, and old engines reduce efficiency.
• Fuel quality: Lower-grade fuels may reduce engine efficiency.

For accurate estimates, use your actual observed fuel economy from recent trips, not the manufacturer’s advertised figure.

Step-by-Step Fuel Cost Estimation Process

1. Determine your baseline fuel economy:

   • Reset your trip computer after a full tank.
   • Drive normally for 2–3 tanks.
   • Note the average MPG or L/100 km.

2. Plan your route:

   • Use Google Maps or similar to get the total distance.
   • Note elevation changes or heavy traffic areas.

3. Adjust for conditions:

   • Add 10–20% to fuel consumption for highway speeds, hills, or AC use.
   • Add 20–30% for city driving with frequent stops.

4. Research fuel prices:

   • Use apps like GasBuddy (US) or PetrolPrices (UK) to find average prices along your route.
   • Account for regional price differences (e.g., urban vs. rural, state/province taxes).

5. Calculate total cost:

   • Apply the formulas above.
   • Include a 5–10% buffer for unexpected detours or price fluctuations.

Pro Tips & Best Practices for Reducing Fuel Costs

• Drive smoothly: Maintain steady speeds and anticipate traffic to avoid hard braking.
• Observe speed limits: Reducing highway speed from 75 mph to 65 mph can improve MPG by 10–15%.
• Use cruise control: On highways, it maintains constant speed and saves fuel.
• Reduce weight: Remove unnecessary items from your trunk or roof rack.
• Maintain your vehicle: Keep tires inflated to the recommended PSI and follow service schedules.
• Plan fuel stops: Fill up in areas with historically lower prices (e.g., near refineries or in competitive markets).
• Consider alternative routes: A slightly longer highway route may be cheaper than a shorter city route.

Practical Applications

• Trip budgeting: Include fuel as a line item in your travel budget.
• Vehicle comparison: Calculate annual fuel costs when choosing between cars.
• Fleet management: Optimize routes and driver training to reduce operational costs.
• Commuting analysis: Compare the cost of driving vs. public transport or carpooling.
• Environmental impact: Estimate CO₂ emissions using fuel consumption (1 gallon ≈ 8.9 kg CO₂).