How to Calculate Gas Mileage: Frequently Asked Questions

Gas mileage, measured in miles per gallon (MPG), is calculated by dividing the number of miles you drove by the number of gallons of fuel you used. The formula is:

For example, if you drove 350 miles and used 12.5 gallons, your gas mileage would be 350 ÷ 12.5 = 28 MPG. As Calculator Soup explains, you simply divide the distance traveled by the fuel consumed to get your fuel economy in whatever unit system you prefer.

You can skip the manual math entirely by using our free gas mileage calculator, which computes MPG, L/100km, and cost per mile instantly.

The fill-up method (also called the tank-to-tank method) is the most accurate way to calculate gas mileage at home, recommended by sources from Care4Air to the U.S. Department of Energy. Here is how it works:

1. Fill your tank completely until the pump clicks off. Do not "top off" after the click.
2. Reset your trip odometer to zero (or note your current odometer reading).
3. Drive as you normally would until it is time to refuel.
4. Fill up again completely at the same pump if possible. Record the gallons displayed on the pump or receipt.
5. Record your trip miles from the trip odometer.
6. Divide miles by gallons. The result is your MPG for that tank.

Drivers in the Reddit r/answers community overwhelmingly agreed that this manual method is the most reliable approach, with many noting it is the same technique mechanics and automotive journalists have used for decades.

The MPG formula depends on which unit system you are using:

For metric users, fuel economy is typically expressed as liters per 100 kilometers:

To convert between these metrics: MPG = 235.215 ÷ L/100km and vice versa. Calculator Soup provides conversions for all four common fuel economy metrics: MPG (US), MPG (Imperial), km/L, and L/100km.

Calculating MPG by hand requires just two numbers: miles driven and gallons used. Here is a worked example from Care4Air:

Scenario: You fill up your tank and reset your trip odometer. Over the next week, you drive 337 miles. When you fill up again, the pump shows you added 12.5 gallons.

If you prefer metric units, Care4Air recommends dividing miles by liters first to get miles per liter, then multiplying by 4.544 to convert to miles per Imperial gallon. For example: 160.1 miles ÷ 22.3 liters = 7.18 miles per liter × 4.544 = 32.6 MPG (Imperial).

For reliable gas mileage data, track at least 3 to 5 consecutive fill-ups and average the results. Automotive experts at Airport Marina Honda recommend consistent tracking over time because a single tank can be heavily influenced by unusual driving conditions.

Experienced drivers on Reddit echoed this advice, with one commenter reporting their MPG varied by as much as 8 MPG between summer highway driving and winter city driving. Some users recommended logging every fill-up with apps like Fuelly or Road Trip to automatically calculate running averages and spot trends.

For example, if your last five tanks gave you 25, 28, 21, 30, and 26 MPG, your average would be (25 + 28 + 21 + 30 + 26) ÷ 5 = 26 MPG.

Dashboard MPG displays are typically optimistic by 5–15%. As Edmunds explains, "these fuel economy gauges can often be overly optimistic because automakers often vary in their approach to programming them."

Your car's onboard computer calculates mileage by counting fuel injection pulses relative to distance traveled, but this method assumes a specific fuel composition. Varying ethanol content (US gasoline can contain 0–15% ethanol), sensor tolerances, and manufacturer programming differences all introduce error.

Multiple drivers on Reddit confirmed that their dashboard MPG was consistently 1–3 MPG higher than their manually calculated figure. The consensus: use the dashboard as a trend indicator (are you driving more or less efficiently right now?), but verify with the fill-up method when precision matters.

The MPG number on your car's window sticker is an EPA lab estimate, not a real-world guarantee. EPA testing is conducted on a dynamometer (a treadmill for cars) under controlled conditions that may not match your driving reality.

Several factors cause real-world MPG to differ from EPA estimates:

• Driving style: Aggressive acceleration and hard braking can reduce fuel economy by 15–30% in city driving.
• Terrain: Hilly routes use significantly more fuel than flat roads.
• Climate: Cold weather, AC use, and winter-blend gasoline all reduce efficiency.
• Vehicle condition: Underinflated tires, dirty air filters, and deferred maintenance hurt MPG.
• Speed: EPA highway tests top out at moderate speeds; real-world highway driving at 70+ MPH reduces fuel economy significantly.

The EPA itself notes that their estimates assume 15,000 miles of driving per year and a 55/45 city/highway split. Your actual mileage will vary based on how and where you drive.

Using the same pump is not strictly necessary, but it improves measurement consistency. Several experienced drivers in the Reddit r/answers thread recommended using the same gas station and ideally the same pump to minimize fill-level variation between measurements.

The reason: different pumps can have slightly different automatic shutoff sensitivities, meaning one pump may stop sooner or later than another. This means the "full" level varies, which affects how many gallons you record for each fill-up. Over multiple tanks, these small differences average out, but for any single tank measurement, the same pump gives a more apples-to-apples comparison.

According to the EPA's testing documentation, all fuel economy testing is performed in a laboratory on a dynamometer using five standardized test cycles:

The combined MPG shown on the window sticker uses a harmonic average of 55% city and 45% highway:

The FuelEconomy.gov trip calculator lets you look up EPA ratings for specific vehicles and estimate real-world trip costs based on these ratings.

L/100km (liters per 100 kilometers) is the standard fuel economy measurement used in Canada, Europe, and Australia. Unlike MPG where higher numbers are better, with L/100km lower numbers mean better fuel economy (you are using less fuel per distance).

To convert between US MPG and L/100km, use this formula:

For example, a car getting 30 MPG uses approximately 235.215 ÷ 30 = 7.84 L/100km. To convert from Imperial MPG, use 282.481 instead of 235.215. Calculator Soup identifies L/100km as one of the four primary fuel economy metrics used worldwide, alongside US MPG, Imperial MPG, and km/L. Our gas mileage calculator shows both MPG and L/100km automatically.

A US gallon is 3.785 liters, while an Imperial gallon is 4.546 liters—about 20% larger. This means a car rated at 30 MPG using US gallons would be rated approximately 36 MPG using Imperial gallons, even though the car's actual efficiency is identical.

The US gallon is used in the United States, while the Imperial gallon is used in the United Kingdom and some Commonwealth countries. When comparing fuel economy numbers across countries or reading manufacturer specs from a non-US source, always check which gallon standard is being referenced. As Calculator Soup notes, confusing the two can lead to significant misunderstandings when comparing vehicles.

The average new vehicle sold in 2024 achieved 27.2 MPG, according to the EPA's 2025 Automotive Trends Report. Generally speaking:

• 30+ MPG is considered fuel-efficient for a gasoline car
• 40+ MPG is excellent and typically found in hybrids and small efficient sedans
• 50–60 MPG is standard for most hybrid vehicles
• 100+ MPGe is possible with plug-in hybrids running on electricity

However, "good" is relative to vehicle type. A pickup truck averaging 22 MPG is actually performing well for its class, while a compact sedan averaging 22 MPG would be underperforming. Check the average MPG by vehicle type table in our in-depth guide for detailed category breakdowns.

Here are the latest real-world fuel economy averages from the EPA's 2025 Automotive Trends Report for model year 2024:

If your calculated MPG is significantly below your vehicle category average (3–5 MPG lower than expected), it may signal maintenance issues such as underinflated tires, a dirty air filter, or a malfunctioning oxygen sensor.

Yes, running your air conditioning reduces fuel economy, particularly at low speeds. According to the U.S. Department of Energy, AC use can reduce a conventional vehicle's fuel economy by more than 25% in certain conditions. This effect is significant enough that the EPA includes a dedicated SC03 test cycle that simulates driving with AC at 95°F to factor this into the window sticker estimate.

At highway speeds, AC is generally more fuel-efficient than opening windows (which increases aerodynamic drag). In city driving, rolling down windows at low speeds typically uses less fuel than running the AC compressor.

Cold weather reduces gas mileage for multiple compounding reasons, according to FuelEconomy.gov:

• Engine warm-up time: Engines take longer to reach their efficient operating temperature in cold weather, burning more fuel during the warm-up period.
• Tire pressure drops: Tires lose approximately 1 PSI per 10°F temperature decrease, increasing rolling resistance.
• Winter-blend gasoline: Winter fuel formulations contain slightly less energy per gallon than summer blends.
• Heating and defrost: Running the heater, heated seats, and rear defroster draws extra energy.
• Thicker fluids: Engine oil and transmission fluid are thicker in cold temperatures, increasing friction.

Combined, these factors can reduce fuel economy by 15–30% on short trips (under 5 miles), where the engine never fully warms up. On longer drives, the effect is typically 5–10%.

Fuel economy drops significantly at higher speeds due to increased aerodynamic drag. The U.S. Department of Energy estimates that driving at 70 MPH instead of 55 MPH can lower your fuel economy by 15% or more.

Most vehicles reach their peak fuel efficiency between 45–55 MPH. Above 50 MPH, every 5 MPH increase is roughly equivalent to paying an additional $0.20–$0.30 per gallon for gas. Aerodynamic drag increases with the square of speed, meaning doubling your speed quadruples the drag force your engine must overcome.

Yes, tire pressure has a direct and measurable impact on gas mileage. According to the U.S. Department of Energy, underinflated tires increase rolling resistance and can reduce fuel economy by up to 3%. Specifically, you lose approximately 1% fuel economy for every 2 PSI drop across all four tires.

Tires naturally lose about 1 PSI per month and an additional 1 PSI per 10°F temperature drop. That means a 40°F temperature swing from summer to winter can cost you 4 PSI and roughly 2% of your gas mileage. Check your tire pressure at least monthly and always inflate to the pressure listed on the driver's door jamb (not the maximum shown on the tire sidewall).

On flat highway roads, yes—cruise control improves gas mileage by maintaining a constant speed and eliminating the small, unconscious speed variations that waste fuel. Consumer Reports and the Department of Energy both recommend cruise control for sustained highway driving.

However, on hilly terrain, cruise control can actually hurt efficiency. The system accelerates hard on uphills to maintain the set speed, using more fuel than necessary. In hilly areas, manually easing off the throttle on uphills and letting gravity assist on downhills is more fuel-efficient. Adaptive cruise control (ACC) systems in newer vehicles are generally better at managing hills.

Once you know your MPG, calculating your fuel cost per mile is straightforward. As Calculator Soup explains:

This number is useful for comparing the true operating cost of different vehicles, estimating commute expenses, or calculating mileage reimbursement. At current national average gas prices, most cars cost between $0.08 and $0.18 per mile in fuel. Use our fuel cost calculator to get your exact cost per mile without any manual math.

To estimate how much fuel a trip will cost, use this formula:

The FuelEconomy.gov trip calculator is an excellent government tool that lets you select your specific vehicle, enter your route, and get fuel cost estimates based on EPA-rated efficiency. You can even compare multiple vehicles side by side for the same trip.

For a quicker calculation, our fuel cost calculator lets you enter your distance, fuel economy, and fuel price to get instant results in both US and metric units.

The most effective ways to improve gas mileage, backed by the U.S. Department of Energy, AAA, and Consumer Reports:

Driving habits:

• Accelerate gently. Aggressive acceleration is the biggest MPG killer—smooth driving can improve city fuel economy by up to 40%.
• Drive 55–60 MPH on highways. Every 5 MPH above 50 costs you more fuel.
• Use cruise control on flat highway stretches to maintain steady speed.
• Coast to stops. Lift off the gas early when approaching red lights.
• Minimize idling. Idling can burn up to half a gallon per hour.

Vehicle maintenance:

• Check tire pressure monthly. Underinflated tires reduce MPG by up to 3%.
• Replace air filters. A dirty air filter restricts airflow and wastes fuel.
• Use recommended motor oil. The right grade can improve MPG by 1–2%.
• Get regular tune-ups. A faulty oxygen sensor alone can cut MPG by 20%.

Reduce weight and drag:

• Remove unnecessary weight. Every 55 lbs extra costs about 1% in fuel economy.
• Remove roof racks when not in use—they increase drag by 2–25% depending on speed.
• Combine trips. A warmed-up engine is more fuel-efficient than a cold one.

For a more detailed breakdown with 12 specific tips, see our complete fuel-saving tips guide.