Gas Mileage Calculator

Calculate your vehicle fuel efficiency.

What Is a Gas Mileage Calculator?

A gas mileage calculator is a tool that determines your vehicle's fuel efficiency by comparing the distance traveled to the amount of fuel consumed. The primary output is miles per gallon (MPG) in the United States, or liters per 100 kilometers (L/100km) in countries that use the metric system. Beyond simple efficiency, the calculator also computes cost per mile, cost per kilometer, and total fuel expenditure for a trip.

Understanding your vehicle's actual fuel economy is important for budgeting transportation costs, comparing vehicles, evaluating driving habits, and planning road trips. Manufacturer estimates provide a baseline, but real-world conditions such as traffic, terrain, weather, and driving style cause significant variation. Tracking your actual mileage over time reveals your true fuel costs and can highlight maintenance issues like clogged air filters or failing oxygen sensors that quietly erode efficiency.

How Gas Mileage Calculations Work

The fundamental formula for fuel efficiency is simple division:

MPG = Distance Traveled (miles) / Fuel Used (gallons)

For metric efficiency, the calculation inverts the relationship:

L/100km = (Fuel Used in liters / Distance in km) x 100

Cost calculations build on these values:

Total Fuel Cost = Fuel Used x Price Per Unit

Cost Per Mile = Total Fuel Cost / Distance in Miles

Cost Per Kilometer = Total Fuel Cost / Distance in Kilometers

The calculator handles unit conversions automatically. One US gallon equals 3.78541 liters. One mile equals 1.60934 kilometers. One UK (Imperial) gallon equals 1.20095 US gallons or 4.54609 liters. These conversions allow you to enter data in whichever units are most convenient and still receive results in both measurement systems.

How to Use This Calculator

Enter the distance traveled. Input the number of miles or kilometers driven. Select the appropriate unit from the dropdown. If you reset your trip odometer at the last fill-up, use that reading for maximum accuracy.
Enter the fuel used. Input the amount of fuel consumed in gallons (US), liters, or UK gallons. For the most precise reading, fill your tank completely both before and after the measurement period, and record the amount needed to refill.
Enter the fuel price (optional). Input the price per gallon or liter to see cost calculations. The price unit should match your fuel unit selection. If you leave this at zero, the calculator will still show efficiency metrics but skip cost breakdowns.
Review the results. The calculator displays your MPG prominently along with the metric equivalents (L/100km and km/L). If you entered a fuel price, a cost breakdown shows total fuel cost, cost per mile, and cost per kilometer. A trip summary confirms the converted distances and fuel volumes.

Worked Examples

Example 1: Standard Commute

You drove 250 miles on a tank and used 8.5 gallons of regular gas at $3.45 per gallon. MPG: 250 / 8.5 = 29.4 MPG. That is equivalent to 8.0 L/100km. Total fuel cost: 8.5 x $3.45 = $29.33. Cost per mile: $29.33 / 250 = $0.12.

Example 2: Road Trip with Metric Units

You drove 620 kilometers and used 52 liters of fuel at $1.65 per liter. L/100km: (52 / 620) x 100 = 8.4. That converts to roughly 28.0 MPG. Total cost: 52 x $1.65 = $85.80. Cost per kilometer: $85.80 / 620 = $0.14.

Example 3: Comparing Two Vehicles

Vehicle A traveled 400 miles on 12.5 gallons (32.0 MPG). Vehicle B traveled 400 miles on 16.7 gallons (24.0 MPG). At $3.50 per gallon, Vehicle A cost $43.75 and Vehicle B cost $58.45 for the same trip. Over 15,000 miles per year, Vehicle A saves roughly $548 in fuel annually compared to Vehicle B.

Example 4: UK Imperial Gallons

You drove 180 miles on 6.2 UK gallons of diesel at 1.45 pounds per liter. Since 1 UK gallon equals 4.546 liters, you used 28.2 liters. MPG (Imperial): 180 / 6.2 = 29.0. MPG (US): 180 / 7.44 = 24.2. The Imperial gallon is larger, so Imperial MPG is always higher than US MPG for the same vehicle.

Tips for Improving Gas Mileage

Maintain steady speeds on the highway. Rapid acceleration and hard braking can reduce fuel economy by 15 to 30 percent at highway speeds and 10 to 40 percent in stop-and-go traffic. Use cruise control on flat highways to maintain consistent speed.

Keep your tires properly inflated. Check pressure monthly using a reliable gauge when tires are cold. Properly inflated tires reduce rolling resistance and can improve mileage by up to 3 percent. Consider low-rolling-resistance tires when it is time for replacement.

Reduce excess weight and aerodynamic drag. Remove roof racks, cargo carriers, and unnecessary heavy items from your vehicle. Every 100 pounds of extra weight reduces MPG by approximately 1 percent. Roof-mounted cargo boxes can decrease fuel economy by 10 to 25 percent at highway speeds.

Track your mileage consistently. Record fuel purchases and odometer readings each time you fill up. Over several tanks, you will establish a reliable baseline. Sudden drops in efficiency can signal maintenance needs like a dirty air filter, failing spark plugs, or a malfunctioning oxygen sensor.

Plan and combine trips. A warmed-up engine is significantly more fuel-efficient than a cold one. Combining multiple short errands into one trip allows the engine to stay at operating temperature and reduces total miles driven. Route planning to avoid traffic congestion also saves fuel.

Use the recommended fuel grade. If your vehicle is designed for regular 87-octane fuel, using premium provides no benefit and wastes money. Conversely, if premium is required, using regular can reduce performance and efficiency while potentially causing engine damage over time.

Frequently Asked Questions

How do I calculate my car's miles per gallon (MPG)?

Divide the total miles driven by the gallons of fuel used. For example, if you drove 300 miles and used 10 gallons of gas, your car gets 30 MPG. The most accurate method is to fill your tank completely, reset your trip odometer, drive normally until you need to refuel, fill up again, and divide the miles driven by the gallons it took to refill.

What is the difference between MPG and L/100km?

MPG (miles per gallon) is the standard fuel efficiency measure in the United States and measures how far you can travel on one gallon of fuel. L/100km (liters per 100 kilometers) is the standard in most other countries and measures how much fuel is needed to travel 100 kilometers. A higher MPG is better, while a lower L/100km is better. To convert MPG to L/100km, divide 235.215 by the MPG value.

What is considered good gas mileage?

For conventional gasoline cars, 30 MPG or higher is generally considered good. Compact cars often achieve 30 to 40 MPG, while midsize sedans typically get 25 to 35 MPG. SUVs and trucks usually range from 18 to 28 MPG. Hybrid vehicles can reach 40 to 60 MPG, and plug-in hybrids may exceed 100 MPGe in electric mode. The EPA provides official fuel economy ratings for all new vehicles sold in the United States.

Why does my actual MPG differ from the manufacturer's estimate?

EPA estimates are measured under standardized laboratory conditions that may not reflect real-world driving. Factors that reduce actual MPG include aggressive acceleration and braking, highway speeds above 50 mph (aerodynamic drag increases exponentially), cold weather (engine takes longer to reach optimal temperature), short trips (engine never fully warms up), heavy cargo, underinflated tires, using air conditioning, and mountainous terrain.

How does driving speed affect fuel economy?

Fuel economy drops significantly at higher speeds. Most vehicles achieve peak efficiency between 35 and 45 mph. Above 50 mph, every 5 mph increase is roughly equivalent to paying an additional $0.20 to $0.30 per gallon of gas. At 70 mph, aerodynamic drag is roughly double what it is at 50 mph. Cruise control helps maintain a steady speed and typically improves highway fuel economy by 7 to 14 percent.

Does tire pressure affect gas mileage?

Yes, underinflated tires increase rolling resistance and reduce fuel economy. For every 1 PSI drop in pressure across all four tires, gas mileage decreases by about 0.1 to 0.3 percent. Tires that are 10 PSI underinflated can reduce fuel economy by 3 percent or more. Check tire pressure monthly when tires are cold, and inflate to the pressure listed on the driver's door jamb sticker, not the maximum pressure printed on the tire sidewall.

How do I calculate cost per mile for my vehicle?

Divide your total fuel cost by the miles driven. If you spent $42 on gas and drove 300 miles, your fuel cost per mile is $0.14. For total operating cost per mile, also factor in insurance, maintenance, depreciation, and registration divided by annual miles driven. The average total cost of vehicle ownership in the United States is approximately $0.60 to $0.80 per mile depending on the vehicle type.

Is premium gas worth the extra cost for fuel efficiency?

Unless your vehicle's manufacturer specifically requires premium fuel, using it will not improve fuel economy or performance. Modern engines with standard compression ratios are designed to run on regular 87-octane gasoline. Premium fuel only benefits engines with high compression ratios or turbochargers that are designed for it. Using regular fuel in a premium-required vehicle can cause engine knocking and reduced performance.