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
- Fill your tank completely and note the odometer reading
- Drive normally until you need to refuel
- Fill the tank again completely — note the gallons added
- 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
| MPG | L/100km | |-----|---------| | 20 | 11.76 | | 25 | 9.41 | | 30 | 7.84 | | 35 | 6.72 | | 40 | 5.88 |
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
| Vehicle Type | Average MPG | |-------------|-------------| | Compact car | 32–38 | | Sedan | 28–35 | | SUV | 22–28 | | Pickup truck | 18–24 | | Hybrid | 45–55 | | Electric (MPGe) | 100+ |
Use our Gas Mileage Calculator to calculate your MPG, trip costs, and compare fuel efficiency across vehicles.