Gas Mileage Calculator
Calculate fuel efficiency in MPG, km/L, and cost per mile
How It Works
Overview
A gas mileage calculator (also called fuel economy or MPG calculator) tells you how far your vehicle goes on each unit of fuel — and, more usefully, how much that distance is actually costing you. Enter the miles driven since the last fill-up and the gallons it took to refill, and the calculator outputs your real-world MPG plus equivalents in km/L and L/100km. Adding the gas price gives you cost per mile and cost per kilometer.
MPG isn't fixed — it varies between trips and over a vehicle's lifetime. Tracking it over multiple tanks reveals problems before they get expensive: a sudden 10% drop often signals a worn spark plug, dirty filter, or under-inflated tires. It's the simplest health check most drivers can do for their car.
The Formula
US uses MPG (miles per gallon) — higher is better. Most other countries use L/100km (liters per 100 kilometers) — lower is better. Some places use km/L — higher is better. They're all inversions of the same physical quantity.
Cost per mile = price per gallon ÷ MPG. At $3.50/gallon and 30 MPG that's $0.117 per mile, or about $14 for a 120-mile trip.
Worked Example
You filled up to a full tank, drove 350 miles on a road trip, then refilled with 11.2 gallons. Gas was $3.79/gallon.
- MPG = 350 ÷ 11.2 = 31.25 MPG
- L/100km = 235.215 ÷ 31.25 = 7.5 L/100km
- Cost per mile = 3.79 ÷ 31.25 = $0.121
- Cost for the 350-mile trip = $42.45
Over 12,000 miles per year that's ~$1,455 in fuel. A 22 MPG SUV under the same conditions would cost ~$2,066/year — a $611/year difference.
When to Use This
- Tracking real-world fuel economy — the most reliable way to spot maintenance issues early.
- Trip planning — estimating fuel cost for a road trip.
- Buying a car — converting MPG into annual fuel cost differences.
- Reimbursement — calculating mileage compensation for work travel.
- Comparing vehicles — putting hybrids, EVs (via MPGe), and gas cars on the same scale.
Common Mistakes to Avoid
- Using a single trip. A short hilly trip can read 25% off your true average. Track over a full tank — refill to refill.
- Mismatching units. Don't mix US gallons with liters or miles with km. The calculator handles the conversion if you stay within one system.
- Ignoring driving habits. The same car driven by two people can differ by 5+ MPG. Hard acceleration and high cruising speed dominate.
- Comparing summer to winter. Cold weather, winter gas blends, and warm-up time can drop MPG 10–15% in winter. Compare seasonally.
- Trusting the dashboard average. Many cars overstate MPG by 1–3 MPG. The hand calculation from receipts is the truth.
Frequently Asked Questions
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