Gas Mileage Calculator

MPG is one division: miles driven divided by gallons burned

Miles per gallon is not a setting and it is not a guess. It is the distance you covered divided by the fuel you put back in. Two numbers, one division.

Everything else on this page follows from that. Gallons per 100 miles is 100 divided by MPG. Cost per mile is the price of a gallon divided by MPG. Annual fuel cost is the miles you drive in a year divided by MPG, times the price.

The reason people get confused is that the car reports a number that changes every few seconds, while the pump gives you one number for a whole tank. The pump number is the honest one, because it averages out the cold start, the traffic jam and the highway run.

One warning about the arithmetic: MPG is not linear in a way that matches intuition. Going from 10 MPG to 15 MPG saves 200 gallons over 12,000 miles. Going from 30 MPG to 35 MPG saves 57 gallons over the same distance. A 5 MPG improvement is worth about 3.5 times more at the bottom of the scale than at the top, which is why trading a 15 MPG truck for a 20 MPG one matters more than trading a 30 MPG car for a 35 MPG one.

If you want the number that behaves linearly, use gallons per 100 miles. It adds up the way fuel does.

The two fill method, step by step

The method is old, free and more accurate than most phone apps, because it uses the same pump and the same click-off point at both ends.

1. Drive to a station and fill the tank until the pump clicks off. Do not top off after the click.
2. Reset the trip meter to zero, or write down the odometer reading.
3. Drive normally for at least 200 miles, ideally a whole tank.
4. Return to the same station, ideally the same pump, and fill until it clicks off again.
5. Write down the gallons on the receipt and the miles on the trip meter.
6. Divide miles by gallons. That is your MPG for that tank.

The two click-offs matter because the tank is not a measuring cup. A pump that clicks early leaves two gallons of space, and two gallons on a 300 mile tank moves the answer by 20%. Same station, same pump, same click, and the error drops to a fraction of a gallon.

Do not reset the trip meter until after you have written the number down. The most common way to lose a tank of data is to reset it at the start of the next fill and then forget what the last one said.

For a household with two drivers, note who drove and what the tank was used for. A week of school runs and a week of highway driving produce numbers that look like different cars, and knowing which is which is the point of tracking.

Worked example: 300 miles on 10 gallons

You fill up, reset the trip meter, drive 300 miles, and the pump takes 10 gallons at $3.50 a gallon.

• MPG: 300 divided by 10 = 30 MPG
• Gallons per 100 miles: 10 divided by 300, times 100 = 3.33
• Cost of the fill: 10 times $3.50 = $35.00
• Cost per mile: $35.00 divided by 300 = $0.117
• Cost per 100 miles: $0.117 times 100 = $11.67

Now compare that with the sticker. If the car is rated at 28 MPG combined, the tank ran 2 MPG better, which is 7.1% above the rating. If the sticker says 33 MPG, the tank ran 9.1% below it, and that is worth a look at tire pressure and driving habits before blaming the car.

The same arithmetic scales to a year. At 12,000 miles and 30 MPG you buy 400 gallons. At $3.50 that is $1,400. At 25 MPG the same year takes 480 gallons and costs $1,680, so 5 MPG is worth $280 a year at this price. At $4.50 a gallon the same gap is worth $360.

That is the number to use when you are deciding whether a repair or a different car pays. A $280 annual saving takes a long time to repay a $6,000 trade, which is why MPG math rarely justifies a new vehicle on its own.

Why your number differs from the window sticker

The window sticker is a lab result, produced under rules the EPA sets and manufacturers follow. According to the EPA fuel economy testing page, vehicles run five defined test cycles: city, highway, a faster high speed cycle, a hot test at 95 degrees Fahrenheit with the air conditioning on, and a cold test at 20 degrees Fahrenheit with heat and defrost running. The results are weighted into the city, highway and combined numbers on the label.

The EPA confirms about 15% of test results itself and relies on manufacturer data for the rest, auditing and retesting when numbers look off. That process produces a fair comparison between models. It does not reproduce your commute.

What the lab cannot see:

• Short trips. A 3 mile drive never gets the engine to full temperature, and cold enrichment burns extra fuel the whole way.
• Speed. Drag rises with the square of speed, so a 75 mph highway run uses noticeably more fuel than the 65 mph test.
• Weather. Cold air is denser, winter fuel blends carry less energy, and a headwind is a load.
• Accessories. Air conditioning can cost a few MPG in city driving, less on the highway.
• Load. Four adults and a trunk full of luggage weigh something, and every pound costs fuel to move.

A gap of 10% to 20% below the sticker is normal. A gap of 30% usually points at something: a dragging brake, a failing thermostat, low tire pressure, or a driving pattern that is mostly 2 mile trips.

City, highway and the combined figure

The label gives three numbers and people usually quote the wrong one. City is the stop and go estimate, highway is the steady speed estimate, and combined is a weighted mix that is meant to represent average use.

City mileage is lower for a simple reason: the engine does work that never becomes distance. Every stop converts fuel into heat in the brakes, every idle minute burns fuel at zero MPG, and every acceleration has to be paid for again at the next light. A hybrid recovers some of that through regenerative braking, which is why hybrids show a bigger city gain than highway gain.

Highway mileage is higher because the engine runs at a steady load in its efficient range, with no stops and little idling. It peaks somewhere around 45 to 60 mph for most cars and falls off above that as aerodynamic drag takes over.

This table is the honest way to compare two cars. A 20 MPG vehicle uses half again as much fuel per mile as a 30 MPG one, and twice as much as a 40 MPG one. Saying "10 MPG better" hides that, because the same 10 MPG gap means different things at different points on the scale.

Tire pressure, oil and the small stuff that adds up

Maintenance does not transform fuel economy, but a few items are worth real money over a year, and the fueleconomy.gov maintenance guide puts numbers on them.

The tire numbers are the ones people underestimate. Four tires 8 psi low on a car rated at 30 MPG cost roughly 1.6%, which is about 0.5 MPG, or $22 a year at 12,000 miles and $3.50 a gallon. Not a fortune, but the check is free and the same pressure protects the tires, which cost far more than the fuel.

The air filter line surprises people. On a modern fuel injected engine the computer compensates for reduced airflow, so a dirty filter costs acceleration rather than MPG. Changing it is still worth doing for how the car drives, just not for the fuel bill.

What does move the number is driving style. Smooth acceleration, keeping speed under 70 mph on the highway, and shutting the engine off instead of idling for long waits are worth more than any additive on the shelf.

Track five tanks, not one

One tank is a data point, not a measurement. Fuel level at the click varies by a gallon or so, weather changes week to week, and a single highway trip can lift an average by 3 MPG.

The fix is to track a running total. Add the miles and the gallons across five fills, then divide the totals once. That is a weighted average, and it is much closer to the truth than averaging the five individual MPG figures, because a tank with more miles should count for more.

Worked example. Five fills:

• 310 miles on 10.2 gallons
• 280 miles on 9.4 gallons
• 340 miles on 10.8 gallons
• 295 miles on 9.9 gallons
• 320 miles on 10.5 gallons

Total miles: 310 + 280 + 340 + 295 + 320 = 1,545. Total gallons: 10.2 + 9.4 + 10.8 + 9.9 + 10.5 = 50.8. The average is 1,545 divided by 50.8 = 30.4 MPG.

The individual tanks range from 29.8 to 31.5 MPG, so the average sits in the middle and is steadier than any single figure. Averaging the five MPG values instead gives 30.4, which is close here but drifts when tank sizes vary a lot.

Keep the receipts in the glove box or log them in a note. Five tanks is about 1,500 miles for most drivers, or a month of normal use, which is enough to spot a real change and short enough to remember what you did differently.

When the math misleads: idling, partial fills and bigger tires

The formula is exact, but the inputs can be wrong in ways that produce confident nonsense.

• Idling. Fuel burned while parked counts in the gallons and contributes no miles. A truck that idles 30 minutes a day can lose 1 to 2 MPG over a tank without any change in how it drives.
• Partial fills. If the second fill stops early, the gallons recorded are too low and the MPG comes out too high. Always fill to the click.
• Oversized tires. A tire 3% larger in diameter covers 3% more ground per revolution, while the odometer records 3% fewer miles. The calculated MPG reads low by roughly 3%.
• E85 and other blends. Ethanol carries less energy per gallon, so a flex fuel vehicle running E85 shows about 20% to 30% lower MPG. That is the fuel, not a fault.
• Towing. A trailer adds weight and drag, and a truck that gets 20 MPG empty can drop to 12 MPG at highway speed with a loaded trailer.

None of these break the division. They change what the division means. A tank that includes 4 hours of idle is not a measurement of highway efficiency, and a tank with a trailer is not comparable to one without.

Write one note next to each number: what the tank was used for. Six months of notes will tell you more about your real fuel cost than any single reading, and it makes the next car decision an arithmetic problem instead of a guess.