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.