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Gas Mileage Calculator

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Fuel Economy (MPG)

The classic US fuel economy calculation: miles driven divided by gallons used. Fill up at the pump, drive until empty (or until your next fill-up), then divide. The tank-fill method is the most accurate way to measure real-world MPG.

MPG = distance / fuel

Distance

How far you can drive on a known amount of fuel — useful for trip planning, range estimates on a partial tank, or checking whether a leg of a road trip can be made without refueling.

distance = MPG × fuel

Fuel Needed

How many gallons (or liters) you'll burn over a known route. Useful before a long drive to estimate refueling stops, or to budget the gallons used between scheduled maintenance intervals.

fuel = distance / MPG

Trip Cost

Total fuel cost for a trip: gallons used multiplied by price per gallon. The single most useful calculation for road-trip budgeting, ride-share margin checks, and comparing routes that differ in distance.

cost = (distance / MPG) × price

How It Works

Fuel economy in the United States is reported as miles per gallon (MPG) — the distance a vehicle travels on one US gallon of fuel. The defining equation is MPG = miles / gallons, and its inverses give you the distance you can drive on a known amount of fuel (distance = MPG × fuel) and the fuel needed for a known distance (fuel = distance / MPG). Trip cost layers a price per gallon on top: cost = (distance / MPG) × $/gallon. Outside the US, most countries report fuel economy as either kilometers per liter (km/L) or — more commonly in Europe — liters per 100 kilometers (L/100km), which is the inverse: lower is better. This calculator handles all three and converts between them automatically.

Example Problem

You fill your car's tank to the top, drive 350 miles, then fill up again — it takes 14 gallons to top off. What is your fuel economy in MPG, and what would a 300-mile trip cost at $3.50 per gallon?

  1. Identify the inputs: distance = 350 mi, fuel = 14 gal.
  2. Write the fuel economy formula: MPG = distance / fuel.
  3. Substitute the values: MPG = 350 / 14.
  4. Evaluate: MPG = 25.0 (US miles per gallon).
  5. For the trip cost, write: cost = (distance / MPG) × price.
  6. Substitute: cost = (300 / 25) × $3.50 = 12 gal × $3.50/gal.
  7. Evaluate: cost = $42.00. The trip burns 12 gallons of fuel and costs $42 at $3.50/gal.

MPG is a US convention — most other countries report L/100km (European) or km/L (Japan, Latin America). 25 MPG equals about 9.4 L/100km or 10.6 km/L. The conversion is exact: 1 mpg (US) = 0.425144 km/L, and L/100km = 235.215 / mpg.

When to Use Each Variable

  • Solve for MPG — after a tank-fill measurement (drive, refuel, divide miles by gallons) to find your real-world fuel economy.
  • Solve for Distance — when planning a trip with a known fuel amount — how far you can drive on a partial tank, or how much range a fuel can provides.
  • Solve for Fuel — before a road trip to estimate gallons needed, or to budget refueling stops on a long route.
  • Solve for Trip Cost — to budget gas money for a road trip, compare driving costs between routes, or check ride-share break-even.

Key Concepts

Three fuel-economy conventions are in worldwide use. The US uses miles per US gallon (MPG, where higher is better). Most of Europe uses liters per 100 kilometers (L/100km, where lower is better — a 5 L/100km car is more efficient than a 9 L/100km car). Japan, much of Latin America, and India use kilometers per liter (km/L, where higher is better). The same physical fuel economy can be reported in all three: 25 mpg (US) ≈ 10.63 km/L ≈ 9.41 L/100km. Importantly, MPG and L/100km are inverses, not linear scales — going from 20 to 25 MPG saves more fuel than going from 40 to 45 MPG, because L/100km drops further on the low end. This is why fuel economy regulations in Europe target L/100km directly: it tracks fuel saved per distance, not distance gained per fuel.

Applications

  • Tank-fill MPG measurement — the gold standard for real-world fuel economy
  • Road-trip budgeting — estimate gas costs before a long drive
  • Comparing EPA estimates to real-world performance — real numbers are typically 10-15% lower
  • Ride-share and delivery margin analysis — net earnings minus gas cost per mile
  • Vehicle purchase decisions — annual fuel cost based on expected miles driven
  • Fleet management — per-mile fuel cost tracking across a vehicle fleet
  • Cross-country trip planning — fuel stops at known intervals

Common Mistakes

  • Mixing US and Imperial gallons. A US gallon is 3.78541 L; an Imperial (UK) gallon is 4.54609 L, about 20% larger. UK 'MPG' figures look better than the same car's US MPG for this reason alone.
  • Trusting the dashboard MPG readout. Most onboard computers estimate fuel use indirectly and tend to overstate MPG by 5-10% versus a tank-fill measurement.
  • Using EPA window-sticker numbers for real-world budgeting. Real-world MPG is typically 10-15% lower than the EPA combined figure, more in cold weather, more for short trips that never reach engine operating temperature.
  • Forgetting that L/100km is inverted. Going from 9 to 8 L/100km saves more fuel per 1000 km than going from 6 to 5 L/100km, even though both 'look like' a 1 unit improvement.
  • Using only one fill-up to compute MPG. Variation between fill-ups is large; average at least 3-5 tanks for a stable real-world MPG number.
  • Mixing units in trip-cost calculations. If you enter distance in kilometers but MPG in mpg (US), the result is wrong — convert both into the same unit system first.

Frequently Asked Questions

How do I calculate MPG?

Divide the miles you drove by the gallons of fuel used: MPG = miles / gallons. The most accurate way is the tank-fill method: fill the tank completely, reset the trip odometer, drive normally, then refill the tank — divide the trip distance by the gallons it took to top off. Example: 350 miles on 14 gallons = 25 MPG.

What is the formula for fuel economy?

In the US: MPG = distance (miles) / fuel (US gallons). In Europe: L/100km = (fuel in liters / distance in km) × 100. In Japan and parts of Asia: km/L = distance (km) / fuel (liters). All three describe the same physical quantity in different units — 25 mpg ≈ 10.63 km/L ≈ 9.41 L/100km.

What is a good MPG?

For a modern sedan, 30+ MPG combined is solid and 40+ MPG is excellent. Compact hybrids routinely hit 50 MPG. SUVs and trucks typically run 18-25 MPG. The 2024 EPA fleet average for new passenger cars was around 28 MPG combined; hybrids average closer to 45-55 MPG. EVs are reported as MPGe (gasoline-equivalent miles per gallon based on energy content), with most modern EVs in the 100-130 MPGe range.

How do I convert MPG to L/100km?

L/100km = 235.215 / mpg (US). The conversion is exact and based on 1 US gallon = 3.78541 L and 1 mile = 1.609344 km. Worked examples: 25 mpg → 235.215 / 25 ≈ 9.41 L/100km; 40 mpg → 235.215 / 40 ≈ 5.88 L/100km; 18 mpg → 235.215 / 18 ≈ 13.07 L/100km. Note this is an inverse relationship: higher MPG produces lower L/100km.

How much will my trip cost in gas?

cost = (distance / MPG) × price per gallon. Example: a 300-mile trip in a 25 MPG car at $3.50/gal costs (300 / 25) × $3.50 = 12 gal × $3.50 = $42.00. Per mile, that's $0.14/mi — handy for ride-share margin checks. For round trips, double the distance. For multi-leg trips with refueling stops, add up segment costs and budget for price variation between stations.

Why is my real-world MPG lower than EPA?

EPA window-sticker numbers come from controlled-condition dyno tests, which don't match real-world driving. Real-world MPG averages 10-15% below the EPA combined figure for most cars, and the gap widens with: cold weather (winter fuel blends, longer warm-up cycles), short trips (engine never reaches operating temperature), aggressive driving (hard acceleration, high cruise speeds), rooftop cargo (drag), and underinflated tires (rolling resistance). Hybrids and EVs are more sensitive to cold than gas cars are.

How do I calculate fuel cost per mile?

Cost per mile = price per gallon / MPG. Example: $3.50/gal ÷ 25 MPG = $0.14/mile. Multiply by your annual miles to estimate yearly fuel cost: 15,000 mi/yr × $0.14/mi = $2,100/yr. This is the single best number for comparing the running cost of two vehicles and the easiest one to project an annual fuel budget from.

Is the tank-fill method accurate?

Yes — it's the most accurate way to measure real-world MPG short of an instrumented vehicle. Fill the tank fully (top off until the pump clicks), record the odometer, drive normally, then refill at the same station (same pump if possible) to the same fill level. The gallons displayed on the pump receipt divided into the distance gives you MPG. Average 3-5 tanks for a stable figure — single-tank numbers vary by 10-20% from weather, traffic, and trip type.

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