How to Calculate Miles Per Gallon — Fuel Efficiency & Trip Cost Guide

Understanding Fuel Efficiency: MPG, L/100km, and What They Mean

Fuel efficiency measures how far a vehicle can travel using a specific amount of fuel. The two most common systems are:

• MPG (Miles Per Gallon): Used in the UK and US (though US gallons differ from imperial gallons).
• L/100km (Litres per 100 Kilometres): Used in Europe, Canada, and most metric countries.

Key difference:

• Higher MPG = better efficiency
• Lower L/100km = better efficiency

The Core MPG Formula

To calculate your car’s real-world MPG, you need two readings from your odometer and your fuel receipt.

• Formula: MPG = Distance Travelled (miles) / Fuel Used (gallons)
• Steps:
  1. Fill your tank completely and reset the trip odometer to zero.
  2. Drive normally until the tank is near empty.
  3. Fill the tank again completely and note:
     • Miles driven (from trip odometer)
     • Gallons added (from the pump receipt)
  4. Divide miles by gallons.

Note for UK users: The UK uses imperial gallons (4.546 litres), not US gallons (3.785 litres). Most UK fuel pumps display litres, so you’ll need to convert.

Converting Between MPG and L/100km

Use these formulas to switch between systems:

• From MPG (UK) to L/100km:
  L/100km = 282.5 / MPG
  (282.5 = 100 km ÷ 1.609 km/mile × 4.546 L/gallon)

• From L/100km to MPG (UK):
  MPG = 282.5 / L/100km

Example:
A car that achieves 40 MPG (UK) uses:
282.5 / 40 ≈ 7.06 L/100km

Calculating Trip Fuel Cost

Once you know your MPG, estimate journey costs:

1. Total distance of trip (e.g., 300 miles)
2. Your car’s MPG (e.g., 35 MPG)
3. Fuel price per litre (e.g., £1.60/L)

Steps:

• Fuel needed (gallons) = 300 ÷ 35 ≈ 8.57 gallons
• Convert to litres: 8.57 × 4.546 ≈ 39 litres
• Total cost = 39 × £1.60 ≈ £62.40

Factors That Affect Real-World MPG

Your calculated MPG may differ from the manufacturer’s claim due to:

• Driving style (aggressive acceleration reduces efficiency)
• Traffic conditions (stop-start vs. motorway cruising)
• Vehicle load (heavy cargo or roof boxes increase drag)
• Tyre pressure (under-inflated tyres increase rolling resistance)
• Engine condition (dirty air filters or old spark plugs reduce efficiency)

Pro Tips & Common Mistakes

• Always use a full tank-to-full tank method—partial fills introduce error.
• Don’t trust the dashboard MPG blindly—it’s often optimistic. Manual calculation is more accurate.
• Track MPG over multiple tanks to get a reliable average.
• Use consistent units: UK MPG ≠ US MPG. Confirm which your car uses.
• For hybrids or EVs, calculate MPGe (Miles Per Gallon Equivalent) using energy consumption data.

Practical Applications

• Compare vehicles before buying
• Monitor engine health (a sudden drop in MPG may indicate a fault)
• Plan long trips with accurate fuel budgets
• Reduce carbon footprint by identifying inefficient driving habits