How to Calculate Gas Mileage (MPG)
Knowing your vehicle's actual fuel economy—rather than the EPA estimate—helps you budget accurately, identify maintenance issues, and compare costs against other transportation options.
The MPG Formula
MPG = Miles Driven / Gallons Used
How to Measure Accurately
- Fill up your tank completely and reset your trip odometer to 0
- Drive normally until you need to refuel
- Fill up again completely; note the gallons pumped
- Divide miles on the odometer by gallons pumped
Example: Trip odometer shows 312 miles; you pump 11.8 gallons. MPG = 312 / 11.8 = 26.4 MPG
Calculating Fuel Cost
Fuel cost = (Miles / MPG) × Price per gallon
Monthly driving 1,200 miles at 26.4 MPG with gas at $3.50/gallon: (1,200 / 26.4) × $3.50 = 45.45 × $3.50 = $159/month
Annual Cost Comparison
| MPG | Cost/Year (15,000 miles @ $3.50) |
|---|---|
| 20 MPG | $2,625 |
| 25 MPG | $2,100 |
| 30 MPG | $1,750 |
| 35 MPG | $1,500 |
| 40 MPG | $1,313 |
Moving from 20 to 30 MPG saves $875/year.
When MPG Drops: Diagnostic Clue
If your calculated MPG drops 10–15% below your usual figure, common causes include:
- Low tire pressure (increases rolling resistance)
- Dirty air filter
- Failing oxygen sensor
- City vs. highway ratio change
- Cold weather (thicker oil, engine warm-up)
Use our MPG calculator to track fuel economy across multiple fill-ups.