How to calculate your MPG and fuel cost

What MPG measures, and why to measure your own

Miles per gallon is distance divided by fuel: how far the car travels on one gallon. The number on a window sticker or a dashboard readout is an average over conditions that may look nothing like your commute, so the only way to know what your car really does is to measure it yourself with fuel you actually paid for.

Measuring has a second payoff. Your measured MPG is the input every cost estimate depends on. If you plug the sticker number into a fuel budget or a road trip plan and your real figure is lower, every dollar estimate comes out too optimistic. A few tanks of honest data fix that for the price of writing two numbers down at the pump.

The fill-to-fill method, step by step

The fill-to-fill method measures fuel by the only reliable gauge you have: what the pump delivers when you top the tank back up to the same level. It does not depend on the fuel gauge, which is deliberately imprecise, or on the trip computer, which is an estimate.

• Fill the tank until the nozzle clicks off the first time, then stop. Do not squeeze in extra. Reset the trip meter to zero.
• Drive normally. A longer stretch is better, because a fixed pump-to-pump variation matters less over 300 miles than over 80.
• Refill at the first click-off, ideally at the same pump or the same kind of pump, and write down the gallons shown.
• Divide the miles on the trip meter by the gallons added. That is the MPG for that tank.
• Repeat for several tanks. Add all the miles and all the gallons separately, then divide once.

A worked example across three tanks

Say you record three consecutive tanks. Tank 1: 318.6 miles on 10.42 gallons is 30.58 MPG. Tank 2: 342.1 miles on 11.05 gallons is 30.96 MPG. Tank 3: 295.4 miles on 9.87 gallons is 29.93 MPG.

Add the totals first: 318.6 + 342.1 + 295.4 = 956.1 miles, and 10.42 + 11.05 + 9.87 = 31.34 gallons. Then divide once: 956.1 / 31.34 = 30.51 MPG. In fuel-per-distance terms, 100 / 30.51 = 3.28 gallons per 100 miles.

The simple average of the three tank figures is 30.49, almost identical here because the tanks were similar. It stops working when they are not. Suppose one tank covered 100 miles on 5 gallons (20 MPG) and another covered 300 miles on 10 gallons (30 MPG). Averaging the two MPG values gives 25, but the real result is 400 miles on 15 gallons, or 26.67 MPG. Always total the miles and total the gallons.

Pump variation is the other trap. If the final fill-up stops 0.5 gallon short of where the first one clicked off, tank 1 would show 318.6 / 9.92 = 32.1 MPG instead of 30.6, roughly 5 percent too high from a half gallon of pump difference. That is why the same click-off point matters, and why measuring over more than one tank helps the errors average out.

L/100 km and converting between units

Most of the world measures fuel economy as fuel per distance: liters per 100 kilometers. The formula is L/100 km = liters used / kilometers driven x 100. A lower number is better, the opposite of MPG.

Worked example: 520 km driven on 38.4 liters. 38.4 / 520 x 100 = 7.38 L/100 km. The reciprocal view, kilometers per liter, is 520 / 38.4 = 13.54 km/L. To compare with a US car, divide 235.215 by the L/100 km figure: 235.215 / 7.38 = 31.85 MPG.

Going the other way, 30 MPG is 235.215 / 30 = 7.84 L/100 km. The Fuel Economy Converter does these conversions, including km/L, so you do not have to repeat the arithmetic.

One caution when reading foreign specifications: a UK imperial gallon is larger than a US gallon, so imperial MPG figures run higher than US MPG for the same car. Check which gallon a number uses before comparing.

The MPG illusion: why equal MPG gains are not equal savings

MPG feels like a straight scale, but it is not. Fuel used is distance divided by MPG, so the relationship is a curve. The EPA's own label guidance on FuelEconomy.gov notes that MPG can be misleading when comparing improvements: over 1,000 miles, going from 10 to 15 MPG saves about 33 gallons, while going from 30 to 35 MPG saves only about 5. Checking the arithmetic: 1,000 / 10 - 1,000 / 15 = 33.3 gallons, and 1,000 / 30 - 1,000 / 35 = 4.8 gallons.

Expressed per 100 miles, the picture is clear: 10 MPG burns 10.0 gallons, 15 MPG burns 6.67, 30 MPG burns 3.33, and 35 MPG burns 2.86. Gallons per 100 miles rises and falls in a straight line with fuel actually burned, which is why the EPA recommends thinking in consumption terms.

Put dollars on it. Over 12,000 miles with gas at 3.50 dollars a gallon, moving from 15 to 20 MPG cuts fuel from 800 to 600 gallons, saving 200 gallons or 700 dollars. Moving from 40 to 45 MPG cuts fuel from 300 to 266.7 gallons, saving 33.3 gallons or about 117 dollars. The same 5 MPG improvement is worth six times as much on the thirstier vehicle.

The illusion also breaks household averages. Two cars each driven 10,000 miles, one at 20 MPG and one at 50 MPG, burn 500 + 200 = 700 gallons for 20,000 miles. That is 28.57 MPG overall, not the 35 you would get by averaging the two ratings.

Real-world mileage versus the EPA rating

EPA ratings come from standardized testing and are best used to compare one vehicle against another, not to predict your own tank. On the label, the combined figure blends city and highway, weighting city 55 percent and highway 45 percent, according to FuelEconomy.gov. Blending is itself an MPG trap: with a city rating of 26 and a highway rating of 34, a straight average gives 29.6, but weighting the fuel burned per mile gives 1 / (0.55 / 26 + 0.45 / 34) = 29.1.

Then reality arrives. FuelEconomy.gov reports that aggressive driving can lower gas mileage by roughly 15 to 30 percent at highway speeds, which would turn a 30 MPG car into one delivering 25.5 down to 21.0. An extra 100 pounds in the vehicle can cost about 1 percent, taking 30 MPG to about 29.7. In city driving, a conventional gasoline car's mileage is roughly 15 percent lower at 20 degrees Fahrenheit than at 77 degrees, and it can drop as much as 24 percent on short 3 to 4 mile trips: 30 MPG becomes 25.5 and 22.8 respectively.

What does a gap cost? Say the combined label says 28 MPG but your fill-to-fill average is 24, which is 14.3 percent lower. At 12,000 miles a year that is 500 gallons instead of 428.6, or 1,750 dollars instead of 1,500 at 3.50 a gallon. The 250-dollar difference is exactly the error you would carry into a budget if you trusted the sticker.

Common mistakes and when the estimate breaks down

Most bad MPG numbers trace back to a handful of habits, and a few situations make any single measurement unreliable no matter how carefully you do it.

• Topping off after the click-off: extra fuel in the filler neck inflates the gallons at one fill-up and skews the tank.
• Using one tank as the answer: a single tank mixing a highway trip with a week of stop-and-go can be off from your true average. Use three or more.
• Trusting the trip computer without a check: it is a convenient estimate, but the hand calculation based on paid-for fuel is the reference to calibrate it against.
• Averaging MPG numbers instead of totaling miles and gallons: as shown above, this overstates the result whenever tanks differ.
• Ignoring tire size changes: bigger tires make the odometer under-count miles. A 2.3 percent larger diameter, for example, makes calculated MPG read about 2.3 percent low, because the odometer is still calibrated to the old tires.
• Comparing across gallon types: US and imperial gallons differ, and an imported spec sheet may use either.
• Seasonal effects: winter, short trips, idling, roof racks, and low tire pressure all pull the number down, so compare tanks from similar conditions.

Turning MPG into a fuel budget

Once you trust your MPG, cost is one more step: fuel cost = (distance / MPG) x price per gallon. At 30 MPG and 3.50 dollars a gallon, each mile costs 3.50 / 30 = 0.1167 dollars, about 11.7 cents. Driving 12,000 miles uses 12,000 / 30 = 400 gallons and costs 1,400 dollars.

Use your measured figure rather than the label one for anything you intend to budget. The Fuel Cost Calculator applies this formula directly, and the Road Trip Cost Calculator adds lodging, food, and tolls on top of it.