Gas Mileage Calculator — Free Online Fuel Economy Tool

How to Use the Gas Mileage Calculator

1. Enter the distance driven: Type the total number of miles you traveled since your last fill-up. For the most accurate reading, fill your tank completely, reset your trip odometer, drive normally, and record the trip distance when you fill up again. You can also calculate distance by subtracting your starting odometer reading from the current one.
2. Enter the gallons used: Input the total gallons pumped at your most recent fill-up. The fill-up should bring the tank to full so the gallon count accurately reflects the fuel consumed over the recorded distance. The receipt from the gas station shows this number precisely.
3. Enter the gas price per gallon: Type the price you paid at the pump for this fill-up. This allows the calculator to determine your cost per mile and cost per kilometer, which are valuable metrics for tracking transportation expenses over time.
4. Review your fuel economy: The results panel instantly displays your fuel economy in three formats: MPG (miles per gallon), km/L (kilometers per liter), and L/100km (liters per 100 kilometers). You also see cost per mile and cost per kilometer for budgeting purposes. Compare your result to your vehicle's EPA rating to assess your driving efficiency.

For the most reliable fuel economy tracking, measure over at least three full tanks and average the results. A single tank can be skewed by unusual driving conditions like a long highway trip or heavy city traffic.

Gas Mileage Formula

Variables Explained

• Distance Driven: The total miles traveled between two complete fill-ups. Accuracy depends on using a full-tank-to-full-tank measurement method. Using your trip odometer eliminates errors from estimating partial trips.
• Gallons Used: The total volume of fuel consumed, measured by how much fuel is needed to fill the tank from the point where you started tracking. This must be from a complete fill-up for accuracy.
• MPG (Miles Per Gallon): The primary fuel economy metric in the United States. Higher values indicate better efficiency. The formula divides distance by fuel to show how far you can travel on one gallon.
• km/L (Kilometers Per Liter): The metric equivalent of MPG, commonly used in parts of Asia and South America. The conversion factor from MPG is 0.425144.
• L/100km (Liters Per 100 Kilometers): The standard fuel consumption metric in Europe, Canada, and Australia. Unlike MPG and km/L, lower values are better because this measures fuel used rather than distance achieved.

Step-by-Step Example

Suppose you filled your tank, drove 420 miles, and then refilled with 15.5 gallons at $3.55 per gallon:

1. Calculate MPG: 420 / 15.5 = 27.1 MPG
2. Convert to km/L: 27.1 x 0.425144 = 11.5 km/L
3. Convert to L/100km: 235.215 / 27.1 = 8.7 L/100km
4. Calculate cost per mile: $3.55 / 27.1 = $0.131 per mile

Your vehicle achieved 27.1 MPG on this tank, costing you about $0.13 per mile in fuel. If the EPA rating for your car is 30 MPG combined, you are running about 10% below the estimate, which is normal for real-world driving.

Practical Examples

Example 1: Lisa Tracks Her Sedan's Efficiency

Lisa drives a 2023 midsize sedan with an EPA-rated 32 MPG combined. After a week of mixed city and highway driving, she fills up and records 285 miles on the trip odometer and 10.8 gallons at the pump. Gas cost $3.42 per gallon:

• MPG: 285 / 10.8 = 26.4 MPG
• km/L: 26.4 x 0.425 = 11.2 km/L
• L/100km: 235.215 / 26.4 = 8.9 L/100km
• Cost per mile: $3.42 / 26.4 = $0.130

Lisa's real-world 26.4 MPG is about 18% lower than the EPA rating, which she attributes to mostly city driving with frequent stops and starts in her daily commute. She plans to test her highway MPG on her next road trip.

Example 2: Kevin Compares His Truck's Seasonal MPG

Kevin drives a full-size pickup truck and tracks his MPG each season. In summer, he recorded 380 miles on 18.1 gallons at $3.60 per gallon. In winter, the same driving patterns showed 340 miles on 19.2 gallons at $3.35:

• Summer MPG: 380 / 18.1 = 21.0 MPG (cost: $0.171/mile)
• Winter MPG: 340 / 19.2 = 17.7 MPG (cost: $0.189/mile)
• Seasonal difference: 3.3 MPG (16% drop in winter)

Kevin's data confirms that cold weather significantly impacts fuel economy. The denser cold air, winter-blend gasoline, and longer engine warm-up times all contribute to his 16% winter efficiency drop, which is consistent with Department of Energy findings.

Example 3: Amanda Tests Her Hybrid on a Highway Trip

Amanda owns a hybrid sedan rated at 52 MPG combined. She takes a 560-mile highway trip and fills up with 11.2 gallons at $3.48 per gallon at her destination:

• MPG: 560 / 11.2 = 50.0 MPG
• km/L: 50.0 x 0.425 = 21.3 km/L
• L/100km: 235.215 / 50.0 = 4.7 L/100km
• Cost per mile: $3.48 / 50.0 = $0.070

Amanda achieved 50 MPG on the highway, which is slightly lower than the EPA combined rating because hybrids are actually more efficient in city driving where regenerative braking recaptures energy. Her fuel cost of just 7 cents per mile makes the trip remarkably affordable.

Gas Mileage Reference Table

Cost per mile assumes $3.50/gallon. Actual figures vary by model, driving conditions, and fuel prices.

Tips and Complete Guide

Accurate Fuel Economy Tracking Methods

The most reliable way to track gas mileage is the full-tank-to-full-tank method. Fill your tank completely, noting the odometer reading or resetting the trip odometer. Drive normally through your typical mix of city and highway routes. When you refuel, fill the tank completely again and record the gallons pumped and miles driven. Divide miles by gallons for your MPG. For statistical reliability, repeat this process for at least three consecutive fill-ups and average the results. Single-tank measurements can be misleading due to varying driving conditions, temperature changes, and even slight differences in how full the pump fills the tank.

Factors That Affect Your Gas Mileage

Numerous factors influence real-world fuel economy beyond just your vehicle's engine and transmission. Driving behavior has the largest impact: aggressive acceleration, hard braking, and speeding above 50 mph can reduce fuel economy by 15% to 33%. Vehicle maintenance matters significantly, as worn spark plugs can reduce efficiency by up to 30%, and a dirty air filter can cost 1% to 3% in MPG. Tire pressure is critical since every PSI below the recommended level costs 0.1% to 0.3% in fuel economy. External factors include temperature (cold weather reduces MPG by 12% to 20%), headwinds, road grade, and traffic congestion. Payload also matters, as carrying an extra 100 pounds reduces MPG by about 1% for average sedans. Use our fuel cost calculator to see how different MPG values translate to dollar costs.

Understanding International Fuel Economy Units

Fuel economy is measured differently around the world, which can cause confusion when comparing vehicles internationally. The United States uses MPG (miles per gallon), where higher numbers mean better efficiency. Most of Europe, Canada, and Australia use L/100km (liters per 100 kilometers), where lower numbers mean better efficiency. Parts of Asia use km/L (kilometers per liter), where higher numbers mean better efficiency. A key insight is that the relationship between MPG and L/100km is not linear. Improving from 15 MPG to 20 MPG saves more fuel than improving from 35 MPG to 50 MPG over the same distance. This is because the curve flattens at higher MPG values, a concept known as the MPG illusion that L/100km avoids entirely.

Common Mistakes to Avoid

• Not filling the tank completely: Partial fill-ups make it impossible to know exactly how many gallons correspond to the miles driven. Always fill to the auto-shutoff click for consistent measurements.
• Using the trip computer instead of manual calculation: Many vehicle trip computers overestimate MPG by 5% to 15%. Manual calculation with actual gallons pumped provides more accurate results for budgeting purposes.
• Measuring over too short a distance: Calculating MPG after a single short trip or partial tank is unreliable. Measure over full tanks and average multiple readings for a true picture of your fuel economy.
• Comparing EPA combined vs real-world numbers unfairly: The EPA combined rating is a weighted average (55% city, 45% highway). If your driving is 90% city, compare your actual MPG to the EPA city rating instead.
• Ignoring seasonal variation: Fuel economy naturally drops in winter and improves in summer. Track your MPG seasonally so you are not alarmed by normal winter decreases of 10% to 20%.

Frequently Asked Questions