Fuel Economy Calculator
Calculate your vehicle's fuel economy in miles per gallon (MPG) by dividing the distance you drove by the fuel you consumed. Use it to verify your real-world MPG against the manufacturer's EPA rating, compare driving styles, or estimate fuel cost for a trip.
Last updated: May 2026
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About this calculator
The formula is: MPG = distance traveled (miles) ÷ fuel used (gallons). The result tells you how many miles your vehicle travels per gallon of fuel — higher is better for fuel efficiency and lower cost per mile. The most reliable way to measure is the "fill-to-fill" method: fill the tank completely, reset the trip odometer, drive normally for a tank or two, then refill completely and divide the miles since the last fill by the gallons added. Avoid one-tank measurements, which can be distorted by pump cutoffs and tank-shape variation; average over 2–3 tanks for a stable figure. Edge cases: zero fuel used produces division by zero; very low miles relative to fuel (city traffic, short trips with cold engines, long warm-ups) understates real cruising economy. Real-world MPG typically runs 10–20% below EPA combined ratings because EPA testing uses controlled dynamometer cycles that don't capture aggressive driving, heavy loads, cold weather, ethanol-blended fuel, A/C use, headwinds, or hilly terrain. Outside the US, fuel economy is usually quoted in liters per 100 km (L/100km), where lower is better — the conversion is L/100km = 235.215 ÷ MPG (US gallons). Convert to kilometers per liter with km/L = 0.425 × MPG. Hybrid and electric vehicles use MPGe (miles per gallon equivalent), which expresses electricity consumption in fuel-energy terms: 33.7 kWh = 1 gallon of gasoline by energy content.
How to use
Example 1 — Highway trip. You filled the tank, drove 380 miles to and from a weekend trip, and refilled with 12.5 gallons of gas. Enter 380 for Distance Traveled and 12.5 for Fuel Used. Result: 30.4 MPG. Verify: 380 ÷ 12.5 = 30.4. ✓ Solid highway economy for a mid-sized sedan; close to EPA highway rating, suggesting steady driving without excessive acceleration. Compare against the same calculation for a tank of mostly city driving — the gap reveals how much fuel city stop-and-go costs you. Example 2 — Mixed city driving over a week. Reset trip after fill-up at 245,180 miles. Refilled today at 245,489 miles, adding 14.2 gallons. Distance = 245,489 − 245,180 = 309 miles. Enter 309 and 14.2. Result: 21.8 MPG. Verify: 309 ÷ 14.2 ≈ 21.76. ✓ Lower than highway MPG as expected; city driving with frequent stops, idling at lights, and short trips that never let the engine reach optimal operating temperature typically runs 20–35% below highway MPG.
Frequently asked questions
Why is my real-world MPG lower than the EPA rating?
EPA tests are conducted on a dynamometer using standardized cycles that don't fully capture real-world driving conditions. Real MPG is typically 10–20% lower than the combined EPA figure because actual driving involves aggressive acceleration, sudden braking, A/C use, heating, cold-weather starts (engines burn more fuel until they reach operating temperature), headwinds, hilly terrain, ethanol-blended fuel (E10 reduces MPG by 3–4%), heavy loads, roof racks, low tire pressure, and traffic. Driving style matters most — gentle acceleration, anticipating stops, and steady highway speeds (55–65 mph range is the sweet spot for most cars; above 70 mph fuel economy falls rapidly due to aerodynamic drag rising with the square of speed) can recapture 10–20% of the gap. Hybrids and EVs show smaller real-world vs EPA gaps because their testing protocols are more representative of typical usage.
How do I convert MPG to L/100km or km/L?
L/100km (the standard outside the US, especially in Europe) is inversely related to MPG — lower L/100km is better, higher MPG is better. The conversion is L/100km = 235.215 ÷ MPG (using US gallons). So 30 MPG ≈ 7.84 L/100km, 25 MPG ≈ 9.41 L/100km, 40 MPG ≈ 5.88 L/100km. Note the conversion uses US gallons (3.7854 L), not Imperial gallons (4.546 L) — UK MPG values are higher than US MPG for the same vehicle. To convert from MPG to km per liter: km/L = 0.425 × MPG (US). So 30 MPG ≈ 12.75 km/L. Most modern non-US car spec sheets quote L/100km because the linear-fuel-per-distance framing is mathematically cleaner for trip-cost calculations: a 500 km trip at 8 L/100km uses exactly 40 liters, no division required.
How does fuel economy compare to MPGe for hybrids and EVs?
MPGe (miles per gallon equivalent) is an EPA metric that expresses electric-vehicle energy consumption in gasoline-equivalent terms by converting kWh to gallons of gasoline based on energy content: 33.7 kWh = 1 gallon of gasoline. So a Tesla rated at 130 MPGe means it travels 130 miles on the energy content of 1 gallon — but the actual cost depends on local electricity prices vs. gasoline prices. At typical US electricity rates (~14¢/kWh) and current gasoline prices, an EV at 130 MPGe costs roughly 4–5¢ per mile in energy, vs. 15–20¢ per mile for a 25 MPG gasoline car. For plug-in hybrids, the EPA reports separate ratings: pure-electric range, all-electric MPGe, and gasoline-only MPG, since real-world economy depends on driving patterns and charging habits.
What are the most common mistakes people make tracking fuel economy?
The biggest is measuring on a single tank rather than averaging over 2–3 tanks; pump cutoff variation, tank shape, and ambient temperature can swing a single-tank result by 5–10%. The second is forgetting to reset the trip odometer at fill-up, then trying to estimate distance from memory. The third is comparing seasonal periods without acknowledging that winter MPG can be 15–25% lower than summer due to cold engine operation, winter fuel formulations, snow tires, and frequent heater use. The fourth is using the dashboard's computed MPG figure (often slightly optimistic — by 1–3 MPG) instead of measuring fill-to-fill, which is the ground-truth method. The fifth is comparing across vehicles with different fuel types — diesel has 12% more energy per gallon than gasoline, so a diesel rated at 35 MPG is roughly equivalent to a gasoline car at 31 MPG in energy efficiency. Finally, people often optimize by buying premium fuel for a car that requires only regular, which adds 30–50 cents per gallon without improving MPG measurably.
When should I not use this calculator?
Skip it for electric vehicles — pure-electric driving doesn't consume gasoline, and MPGe (computed from kWh) is the relevant metric. Use a dedicated EV-efficiency calculator that takes kWh/mile or miles/kWh as input. It is the wrong tool for measuring efficiency on a single short trip (under 50 miles) where the engine never reaches full operating temperature; you need at least a full tank for meaningful data. Do not use it during periods of major engine work (recent oil change, new air filter, sensor replacement) without giving the engine 200–500 miles to settle into new behavior. It also doesn't handle plug-in hybrids well, where total-trip MPG depends on the ratio of electric-only miles vs. gasoline miles. For commercial fleet management or detailed efficiency tracking, use a fleet-management platform that automatically logs every fill-up and trip rather than manual calculations.