Gas Mileage Calculator – MPG, Cost per Mile & L/100km
Miles per Gallon (MPG)
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Cost per Mile
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L/100km
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Distance Driven
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Total Fuel Cost
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How Gas Mileage Calculation Works
Gas mileage (also called fuel economy) is a measure of how far a vehicle can travel on a given amount of fuel. In the United States, it is expressed as miles per gallon (MPG). The EPA's FuelEconomy.gov provides official MPG ratings for every new vehicle sold in America, but real-world mileage often differs from these estimates by 5-20% depending on driving conditions and habits.
To measure your actual gas mileage accurately, use the tank-to-tank method: fill your tank completely, record the odometer reading, drive normally, then fill up again and note how many gallons it took. Divide miles driven by gallons used to get your MPG. This calculator automates that math and also converts your result to liters per 100 kilometers (L/100km) for international comparison. According to the U.S. Department of Energy, the average fuel economy for all new vehicles sold in the U.S. is approximately 26 MPG.
The fill-up tracking feature lets you log multiple refueling events to calculate your running average MPG over time. This is far more accurate than a single measurement because driving conditions vary significantly by season, route, and weather. A sudden drop in average MPG can signal maintenance issues like underinflated tires, a clogged air filter, or a failing oxygen sensor. Use our Fuel Cost Calculator to convert your MPG readings into dollar costs for trip planning.
The Gas Mileage Formula
The MPG formula recommended by the EPA and used by this calculator is straightforward:
MPG = Miles Driven / Gallons Used
To convert to metric: L/100km = 235.215 / MPG
To calculate cost per mile: Cost per Mile = (Gallons Used x Price per Gallon) / Miles Driven
Worked example: Odometer start: 50,000 miles. Odometer end: 50,350 miles. Gallons filled: 12.5. Gas price: $3.50/gal. MPG = 350 / 12.5 = 28.0 MPG. L/100km = 235.215 / 28 = 8.40. Total fuel cost = 12.5 x $3.50 = $43.75. Cost per mile = $43.75 / 350 = $0.125.
Key Terms You Should Know
- MPG (Miles Per Gallon): The U.S. standard for fuel efficiency. Higher is better. The EPA tests vehicles on a dynamometer (roller tester) using standardized city and highway driving cycles to produce the window sticker MPG ratings.
- L/100km (Liters per 100 Kilometers): The metric fuel consumption standard used internationally. Lower is better. A vehicle rated at 8 L/100km uses 8 liters of fuel to travel 100 km.
- Cost Per Mile: Total fuel cost divided by miles driven. This metric is essential for mileage reimbursement, fleet management, and comparing the total operating cost of different vehicles. The IRS standard mileage rate for 2026 is $0.70 per mile.
- Combined MPG: A weighted average of city (55%) and highway (45%) fuel economy ratings used by the EPA on window stickers. This represents typical mixed driving for most consumers.
- MPGe (Miles Per Gallon Equivalent): A metric used for electric and plug-in hybrid vehicles that converts electricity consumption to an equivalent amount of gasoline energy (33.7 kWh = 1 gallon equivalent).
Average MPG by Vehicle Type
The table below shows average fuel economy by vehicle class based on EPA data and real-world consumer reports. Actual mileage varies by model year, engine size, and driving conditions. Annual fuel cost assumes 13,500 miles driven at $3.50 per gallon.
| Vehicle Type | City MPG | Highway MPG | Combined MPG | Annual Fuel Cost |
|---|---|---|---|---|
| Compact Sedan | 30 | 38 | 33 | $1,432 |
| Midsize Sedan | 26 | 34 | 29 | $1,629 |
| Compact SUV/Crossover | 25 | 31 | 27 | $1,750 |
| Full-Size SUV | 18 | 24 | 20 | $2,363 |
| Full-Size Truck | 16 | 22 | 18 | $2,625 |
| Hybrid Sedan | 50 | 47 | 49 | $965 |
| Minivan | 20 | 27 | 23 | $2,054 |
Practical Examples
Example 1 -- Daily commuter: A commuter drives 50 miles round trip daily in a sedan averaging 32 MPG. With gas at $3.50/gallon: daily fuel = 50 / 32 = 1.56 gallons, costing $5.47 per day. Over 250 work days per year, that is $1,367 in commuting fuel. Switching to a hybrid averaging 50 MPG would reduce annual fuel cost to $875 -- a savings of $492 per year. Our Commute Cost Calculator factors in insurance, depreciation, and maintenance too.
Example 2 -- Road trip tracking: A family drives from Chicago to Orlando (1,050 miles) in an SUV. Fill-up 1: 350 miles, 14 gallons (25 MPG). Fill-up 2: 380 miles, 16 gallons (23.75 MPG). Fill-up 3: 320 miles, 14.5 gallons (22.07 MPG). Average: 1,050 miles / 44.5 gallons = 23.6 MPG overall. The drop in MPG on the third fill-up (mountain terrain) shows why multi-fill-up tracking is more informative than a single calculation.
Example 3 -- Detecting a maintenance issue: A driver normally averages 28 MPG over 10 fill-ups. The last 3 fill-ups average only 22 MPG -- a 21% drop. Common causes include: underinflated tires (check with a Tire Pressure Calculator), a dirty air filter, failing spark plugs, or a stuck-open thermostat. The DOE estimates that fixing a faulty oxygen sensor alone can improve MPG by up to 40%.
Tips to Improve Your Gas Mileage
- Use cruise control on highways: Maintaining a constant speed avoids the micro-accelerations that waste fuel. According to Natural Resources Canada, cruise control improves highway MPG by 7-14% on flat roads.
- Maintain proper tire pressure: Check tires monthly. Every 1 PSI below the recommended pressure reduces MPG by about 0.2%. A set of tires 5 PSI low costs you roughly 1% in fuel economy across all four tires.
- Drive at moderate speeds: Aerodynamic drag increases with the square of speed. According to the DOE, every 5 mph over 50 mph is like paying an additional $0.20-$0.30 per gallon. Driving 65 mph vs. 75 mph can improve MPG by 10-15%.
- Avoid excessive idling: Idling gets 0 MPG. Turning off your engine when stopped for more than 10 seconds saves more fuel than restarting uses. Modern fuel-injected engines use only 10 seconds worth of fuel to restart.
- Remove roof racks and excess weight: A roof rack adds 2-8% aerodynamic drag even when empty. Every 100 lbs of excess cargo reduces MPG by approximately 1%. Remove unused cargo carriers when not needed.
- Keep up with maintenance: Replace air filters every 15,000-30,000 miles (1-3% MPG improvement), use the recommended motor oil grade (1-2% improvement), and ensure the engine is properly tuned. A check engine light often indicates an emissions or efficiency problem worth addressing immediately.
Seasonal Fuel Economy Patterns
According to the EPA's FuelEconomy.gov, fuel economy varies by 12-24% between summer and winter driving. Cold temperatures (20 degrees F vs. 77 degrees F) reduce fuel economy because engines take longer to reach efficient operating temperature, battery performance decreases, tire pressure drops, and winter-blend gasoline has slightly less energy per gallon. Short trips in winter are especially inefficient -- a 3-5 mile trip in cold weather can reduce MPG by up to 24% compared to the same trip in warm weather. Air conditioning in summer typically reduces MPG by 10-15% in city driving but has minimal effect at highway speeds where the engine has excess power available.
Frequently Asked Questions
How do I calculate gas mileage (MPG)?
To calculate miles per gallon, divide the number of miles driven by the number of gallons of fuel used. For example, 300 miles divided by 10 gallons equals 30 MPG. For the most accurate results, fill your tank completely, reset your trip odometer, drive normally, then fill up again and record the gallons pumped. The EPA recommends this tank-to-tank method as the most reliable way to measure real-world fuel economy because it accounts for your actual driving conditions.
How do I convert MPG to L/100km?
Divide 235.215 by your MPG value. For example, 30 MPG = 235.215 / 30 = 7.84 L/100km. The relationship is inverse: higher MPG means lower L/100km. The conversion factor 235.215 combines the gallon-to-liter ratio (3.785) with the mile-to-kilometer ratio (1.60934). This conversion is useful when comparing U.S. vehicles with international models or when driving abroad. Use our Fuel Efficiency Converter for quick unit conversions.
What is considered good gas mileage for a car?
Good gas mileage depends on vehicle type. Sedans: 30-40 MPG is good. SUVs: 25-30 MPG. Full-size trucks: 20-23 MPG. Hybrids: 40-60 MPG. Plug-in hybrids: 100+ MPGe. According to the EPA, the average fuel economy of all new vehicles sold in the U.S. is approximately 26 MPG. Any vehicle achieving 10% or more above its class average is considered fuel-efficient. The most efficient non-hybrid cars now exceed 40 MPG combined.
Why does my actual MPG differ from the EPA estimate?
Real-world fuel economy typically differs from EPA estimates by 5-20%. City driving with frequent stops can reduce MPG by 15-30% compared to highway cruising. According to the Department of Energy, aggressive acceleration reduces highway MPG by up to 33%. Cold weather (20 degrees F vs. 77 degrees F) can decrease fuel economy by 12-24%. Air conditioning adds 10-15% fuel consumption at low speeds. Underinflated tires reduce MPG by 0.2% per PSI below recommended pressure.
How can I improve my gas mileage?
The most effective improvements include using cruise control on highways (7-14% MPG gain), maintaining proper tire pressure (up to 3% improvement), removing excess weight and roof racks (1-8% improvement), driving at moderate speeds (10-15% better at 65 vs. 75 mph), and keeping the engine properly maintained. The DOE estimates that fixing a faulty oxygen sensor can improve MPG by up to 40%. Regular oil changes with the manufacturer-recommended grade add 1-2%.
What is the most accurate way to track gas mileage over time?
The most accurate method is fill-up-to-fill-up tracking over multiple refueling events. Each time you fuel up, fill the tank completely, record the odometer and gallons, then calculate MPG for that interval. Averaging 5-10 fill-ups accounts for varying conditions and gives a reliable fuel economy picture. This calculator's fill-up history feature does exactly this. A sudden drop in average MPG often signals maintenance issues worth investigating with a mechanic.