Gas Mileage Calculator
Track your vehicle's performance and optimize your travel budget.
Gas Mileage Calculator
Do you know your car's real-world MPG? The sticker number is an estimate — this gas mileage calculator shows you your actual fuel economy based on your last fill-up, and tracks changes over time so you can spot problems early.
What Does This Calculator Do?
Enter the miles you drove and the gallons of gas you used to calculate your actual miles per gallon (MPG). You can also input gas price to see your cost per mile.
How to Use This Calculator
- Fill your tank completely and note your odometer reading (or trip meter).
- Drive normally until your next fill-up.
- At the next fill-up, note the gallons added and the new odometer reading.
- Enter miles driven (odometer difference) and gallons used.
- Optionally enter gas price per gallon to see cost per mile.
- Click Calculate.
MPG Formula
MPG = Miles Driven ÷ Gallons Used
Cost Per Mile = Gas Price Per Gallon ÷ MPG
Monthly Fuel Cost = Cost Per Mile × Monthly Miles Driven
Example Calculation
You drove 320 miles and used 10.5 gallons. Gas price: $3.60/gallon.
- MPG = 320 ÷ 10.5 = 30.5 MPG
- Cost per mile = $3.60 ÷ 30.5 = $0.118/mile
- Monthly cost (1,200 miles): $0.118 × 1,200 = $141.60
What Affects Your MPG?
- Driving speed: Efficiency peaks around 45–55 mph; drops significantly above 65 mph.
- Tire pressure: Under-inflated tires increase rolling resistance and reduce MPG by 0.2–0.4% per PSI below recommended.
- Acceleration style: Hard acceleration wastes fuel; gradual acceleration and coasting improve MPG.
- Air conditioning: AC can reduce MPG by 5–25% in hot weather.
- Vehicle maintenance: A dirty air filter, spark plugs, or old engine oil all reduce fuel efficiency.
Why Use This Calculator?
A sudden drop in your MPG is often the first sign of a maintenance issue — a failing oxygen sensor, clogged fuel injectors, or low tire pressure. Tracking your real MPG over time lets you catch these problems before they become expensive repairs.
Common Mistakes to Avoid
- Only using one fill-up to judge MPG — one data point can be skewed. Track over 3–5 fill-ups for a reliable average.
- Not filling the tank fully — partial fill-ups make the calculation inaccurate. Always fill to the auto-shutoff click for consistency.
- Comparing highway MPG to city performance — city driving always produces lower MPG due to frequent stopping. Compare like-to-like conditions.
Frequently Asked Questions
What is a good MPG for a car?
For a small car, 35+ MPG is good. For a midsize car, 28–35 MPG is respectable. For an SUV, 22–28 MPG is reasonable. Anything below 20 MPG is a heavy vehicle or older model.
How do I improve my gas mileage?
Maintain proper tire pressure, avoid idling, use cruise control on highways, remove unnecessary roof racks, and get regular tune-ups. Driving more gently has the biggest impact.
How does cold weather affect MPG?
Cold temperatures significantly reduce MPG — EPA estimates a 15–24% drop in fuel economy in very cold weather due to engine warm-up time, thicker engine fluids, and battery performance.
What is the L/100km equivalent?
L/100km = 235.21 ÷ MPG. A car that gets 30 MPG uses 235.21 ÷ 30 = 7.84 L/100km.
Conclusion
Knowing your real MPG is the first step to controlling your fuel costs — and spotting car problems early. Track it consistently over multiple fill-ups and you'll have a reliable baseline to compare against.
Related: Fuel Cost Calculator | Mileage Calculator | Tire Size Calculator
Maintenance Tip
Under-inflated tires are one of the most common causes of poor gas mileage. Checking your tire pressure monthly can save you up to 3% in fuel costs.
