Free calculator
Gas Mileage Calculator
The window sticker is an estimate. Your real MPG comes from two numbers you can read yourself: miles driven and gallons added. Enter them below.
Your real fuel economy
0.0
miles per gallon
- Miles driven
- 0
- Fuel cost per mile
- $0.000
- Cost of this tank
- $0.00
- Range on this tank
- 0 mi
How the calculation works
MPG = miles driven ÷ gallons added. In the example, 320 miles divided by 11.2 gallons is 28.6 MPG. The method works for any vehicle and any fuel, and it is the only way to know your real number rather than the EPA estimate.
Getting an accurate reading
- Fill the tank until the pump clicks off. Do not top off — it skews the measurement.
- Reset the trip odometer, or write down the main odometer reading.
- Drive your normal mix of city and highway until the tank is below a quarter.
- Refill at the same station and pump, again stopping at the first click.
- Divide the miles you drove by the gallons you just added.
Repeat for three to five tanks and average the results. A single tank can be off by 2 to 4 MPG because of temperature, pump calibration, and how full the tank actually was.
What changes your MPG
| Factor | Typical effect |
|---|---|
| Highway speed above 65 MPH | −10% to −25% |
| Air conditioning | −5% to −20% |
| Stop-and-go traffic | −15% to −30% |
| Under-inflated tires | −1% to −3% per 5 PSI |
| Roof rack or cargo box | −2% to −8% |
| Cold weather (below 30°F) | −10% to −20% |
| Extra 100 lbs of cargo | −1% to −2% |
From MPG to money
Once you know your real MPG, the rest is arithmetic. At $4.269 per gallon, a 28 MPG car costs about 15 cents per mile; a 20 MPG car costs about 21 cents. Over 13,500 miles a year that gap is roughly $840. Use the cost per mile calculator to see the full picture, and the trip cost calculator for a specific drive.
Frequently asked questions
How do I calculate my real MPG?
Fill the tank, reset the trip odometer, drive normally, then fill the tank again at the same pump and record the gallons added. MPG equals miles driven divided by gallons added. Using the same pump and the same shut-off method reduces error.
Why is my actual MPG lower than the EPA rating?
EPA testing uses lab conditions and a prescribed drive cycle. Real driving adds cold starts, air conditioning, traffic, hills, high speeds, and extra load. A 15 to 20 percent shortfall is common for mostly city driving.
How many miles should I track to get a reliable MPG?
Three tanks is the practical minimum. One tank can be skewed by a partial fill-up or an unusual trip. Five tanks gives a stable average that reflects your normal mix of city and highway driving.
Does MPG or cost per mile matter more?
Cost per mile, because it combines fuel economy with the price you pay. If you are comparing cars, MPG is easier; if you are budgeting, cost per mile is the number that hits your bank account. Use the cost per mile calculator for that view.