How to Calculate Miles Per Gallon (MPG): Formula and Guide
By Jorge Sanchez · April 20, 2026 · 4 min read
Bottom line: MPG = Miles Driven ÷ Gallons Used. Fill up completely, track miles, fill up again, divide miles by gallons pumped. That's your true MPG — more accurate than your dashboard estimate.
The MPG Formula
MPG = Miles Driven ÷ Gallons Used
Annual Fuel Cost = (Miles/Year ÷ MPG) × Price per Gallon
Cost per Mile = Price per Gallon ÷ MPG
How to Measure Your Real MPG (Step by Step)
- Fill your gas tank completely. Note your odometer reading (e.g., 45,230 miles).
- Drive normally until you need fuel again.
- Fill up completely again. Note the new odometer reading (e.g., 45,560) and exactly how many gallons pumped (e.g., 11.2 gal).
- Calculate: (45,560 − 45,230) ÷ 11.2 = 330 ÷ 11.2 = 29.5 MPG
Annual Fuel Cost by MPG
Assuming 15,000 miles/year and $3.50/gallon:
| MPG | Gallons/Year | Annual Fuel Cost |
|---|---|---|
| 15 MPG (truck/SUV) | 1,000 | $3,500 |
| 25 MPG (average sedan) | 600 | $2,100 |
| 35 MPG (efficient car) | 429 | $1,500 |
| 50 MPG (hybrid) | 300 | $1,050 |
The difference between 15 MPG and 30 MPG is $1,750/year. Over 5 years, that's $8,750 in fuel savings — worth considering when buying a vehicle.
Why Your Actual MPG Differs from EPA Estimates
- Driving style: Hard acceleration and braking can reduce MPG by 15–30%
- Speed: Fuel economy drops sharply above 50 mph — each 5 mph over costs about 7–14% more fuel
- AC use: Can reduce MPG by 5–25% in city driving
- Cold weather: Short trips in cold weather reduce MPG significantly (engine inefficiency when cold)
- Tire pressure: Properly inflated tires improve MPG by up to 3%
- Cargo: Each extra 100 lbs reduces MPG by ~1%
MPG vs. L/100km Conversion
L/100km = 235.21 ÷ MPG
MPG = 235.21 ÷ L/100km
Example: 30 MPG = 235.21 ÷ 30 = 7.84 L/100km
Jorge Sanchez · Live Event Production Specialist · CalQpro