How to Calculate Gas Mileage (MPG): Formula and Fuel Cost Tips

Knowing your car's fuel efficiency helps you budget for trips, compare vehicles, and spot engine problems early. Here's how to calculate it accurately.

The MPG Formula

MPG = Miles Driven / Gallons Used

Step-by-Step Calculation

1. Fill your tank completely and note the odometer reading
2. Drive normally until you need to refuel
3. Fill the tank again completely — note the gallons added
4. Note the new odometer reading

Miles Driven = New Odometer - Starting Odometer
MPG = Miles Driven / Gallons Added

Example: You drove 320 miles and added 11.5 gallons:

MPG = 320 / 11.5 = 27.8 MPG

Metric: L/100km

In Europe and most of the world, fuel efficiency is measured in litres per 100 kilometres. Lower is better.

L/100km = (Litres Used / Kilometres Driven) × 100

Example: Used 35 litres over 420 km:

L/100km = (35 / 420) × 100 = 8.33 L/100km

Converting Between MPG and L/100km

L/100km = 235.215 / MPG
MPG = 235.215 / L/100km

Calculating Trip Fuel Cost

Cost = (Miles / MPG) × Price per Gallon

Example: 400-mile trip, 28 MPG, gas at $3.50/gallon:

Gallons needed = 400 / 28 = 14.3 gallons
Cost = 14.3 × $3.50 = $50.05

What Affects Your MPG?

• Speed: Fuel efficiency drops sharply above 60 mph. Every 5 mph over 60 costs roughly 7–14% more fuel.
• Tyre pressure: Under-inflated tyres increase rolling resistance
• Air conditioning: Can reduce MPG by 5–25% in heavy use
• Driving style: Aggressive acceleration and braking reduces efficiency by 15–30%
• Load: Extra weight reduces MPG — remove roof racks when not in use

Average MPG by Vehicle Type

Use our Gas Mileage Calculator to calculate your MPG, trip costs, and compare fuel efficiency across vehicles.