MPG Calculator (Gas Mileage)
Fill the tank, drive, fill it again: miles divided by gallons is your real MPG, which is usually a few ticks below the window sticker and more honest than it. This works from your trip meter or two odometer readings, converts the answer for the rest of the world, and prices what your gas mileage costs you per mile and per year.
The formula, and the fill-to-click convention that makes it work
The trick is that the second fill measures the first tank. Fill until the pump clicks off, drive, then fill until it clicks again: the gallons going in now are, to a good approximation, exactly the gallons you burned. Use the same style of pump and do not top off past the click, because the click level is the measurement. Half a gallon of fill variance on a 14 gallon tank moves the answer by nearly a full MPG, which is why one tank is an estimate and the average of three or four tanks, total miles over total gallons, is the truth.
Worked example
342 miles on the trip meter, 14.1 gallons to fill back up, at $4.15 a gallon:
342 ÷ 14.1 = 24.3 MPG, which is 9.7 L/100km for the rest of the world and 29.1 miles per UK gallon for the British. The fill cost $58.52, the driving cost 17.1 cents a mile in gas, and at a typical 12,000 miles a year this economy runs about $2,050 a year at the pump.
The MPG illusion, and why the metric world measures upside down
Here is the strange property MPG hides. Over the same 10,000 miles, improving a truck from 12 to 15 MPG saves about 167 gallons, while improving a sedan from 30 to 50 MPG saves only 133, even though the second jump sounds enormous and the first sounds trivial. The reason is that gallons burned is 1 over MPG, so equal-looking MPG gains shrink in value as the number climbs; researchers named this the MPG illusion, and it routinely steers people toward the wrong upgrade. Liters per 100 km, the measure most of the world uses, is fuel per distance rather than distance per fuel, so savings read linearly and the illusion disappears. This page prints your number both ways, and the yearly cost line above is the version your wallet understands.
What a sudden MPG drop is trying to tell you
Against your own usual number, a tank that lands two or three MPG low usually has an ordinary explanation: winter gasoline blends cost a few percent, a cold snap costs more, a roof box can cost up to a quarter of highway economy, and a heavy right foot on a fast trip does the rest. A drop that persists across several tanks with none of those present is worth a look at tire pressures first, which cost economy quietly and are free to fix, then at the usual mechanical suspects. The window sticker is a lab number; your own three-tank average is the baseline that means something.
From measurement to money
Once you know your real MPG, the fuel cost calculator prices any trip with it, and the commute cost calculator uses the same arithmetic to weigh the drive to work honestly. If the yearly figure above stings, the EV savings calculator shows what the same miles cost on electricity, in both directions, with the winter losses counted.
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Frequently asked questions
How do I calculate my gas mileage?
Fill the tank until the pump clicks off, reset the trip meter, drive normally, then fill it again at the same kind of pump until it clicks. Miles on the trip meter divided by gallons on the second fill is your MPG for that tank. 342 miles on 14.1 gallons is 24.3 MPG. The fill-to-click convention matters: it makes the second fill measure exactly the gas you used.
What is a good MPG?
The EPA's real-world fleet average is about 27 MPG, so that is the honest middle. Modern hybrids run 45 to 55, ordinary sedans 28 to 36, crossovers 24 to 30, full-size trucks 17 to 22, and anything towing much less. More useful than the average: compare your number to your own car's window sticker, because a big gap from your own baseline means speed, tires, load or a mechanical issue.
Why is my MPG lower than the window sticker?
The sticker comes from standardized lab cycles, and real driving is usually harsher: higher speeds, more aggressive acceleration, cold starts, cargo, roof racks and underinflated tires all cost economy. Fuel economy falls quickly above about 50 mph, so a mostly-highway-at-75 life lands well under a sticker that assumed gentler cycles. A small gap is normal; a sudden drop from your own usual number is the one worth investigating.
How accurate is measuring MPG from one tank?
Noisy, honestly. Pumps click off at slightly different fill levels, and half a gallon of variance on a 14 gallon fill moves the answer by nearly a full MPG. One tank tells you roughly where you are; the average of three or four tanks tells you the truth. Track a few fills and average the total miles over the total gallons rather than averaging the MPGs.
Why do other countries use liters per 100 km instead of MPG?
Because it measures fuel per distance instead of distance per fuel, which makes savings honest. MPG hides a famous illusion: over the same 10,000 miles, improving from 12 to 15 MPG saves about 167 gallons, while improving from 30 to 50 MPG saves only 133, even though the second jump sounds far bigger. In L/100km the first is obviously the larger saving. This page shows your number both ways.
Does premium gas improve MPG?
Not in an engine designed for regular. Premium resists knock in high-compression engines that require it; it does not carry more energy. Paying 21 percent more per gallon for the same mileage just raises your cost per mile. The exceptions are engines whose manuals require premium, which may pull timing and lose a little economy on regular.
What does my MPG cost me per mile?
Gas price divided by MPG. At the current $4.15 average, 24.3 MPG is about 17 cents a mile in gas, and a typical 12,000 mile year runs about $2,050. The same arithmetic shows what an improvement is worth: at that mileage, each additional MPG saves roughly $80 a year at first and less as the number climbs, which is the MPG illusion again, seen from your wallet.
Trip meter or odometer readings, which should I use?
Whichever you actually recorded, and this page takes both. The trip meter is easier if you remembered to reset it at the fill; two odometer readings work when you did not, and photographing the odometer at each fill is the habit that makes the three-tank average painless.