AutoMath

Running Costs ~3 min read

Your Real MPG Isn't the EPA Number

The window sticker is a lab estimate. Measuring your own MPG takes one tank and tells you what driving actually costs — often 10-25% more than the sticker.

The MPG on the window sticker is a useful number for one thing: comparing two cars on a showroom floor. It is not what your car gets. The EPA figure comes from a standardized lab cycle, and your commute, your climate, and your right foot aren’t in it. The good news is that measuring the real number takes exactly one tank of gas.

The measurement

MPG is the simplest math in all of car ownership:

mpg = miles driven / gallons used

Fill the tank, reset the trip odometer, drive your normal mix for a few hundred miles, then refill and read two numbers: the trip miles and the gallons the pump put in. That’s it.

Your numbersSaved on this device only
The tank
For the cost (optional)
Real-world fuel economy

25.0 mpg

9.4 L/100km · 4.00 gal per 100 mi

What that mileage costs
At $4/gal you spend 14.4¢ per mile — $14 per 100 miles. Over 12,000 miles a year that's $1,728.
Cost per mile
14.4¢
Cost per fill
$43
Gallons / year
480 gal
Fuel / year
$1,728

For a steadier figure, average three or four tanks — a single partial fill can skew one reading because pumps cut off differently. If you log every fill, just use total miles ÷ total gallons.

Why the sticker runs high

Real-world economy typically lands 10-25% below the EPA combined number, and the reasons are all things the lab cycle softens or skips:

  • Cold starts and short trips — the engine spends more time inefficient and warming up.
  • Highway speed — aerodynamic drag climbs sharply above ~65 mph.
  • AC, cargo, and roof racks — load and drag the test doesn’t fully capture.
  • Your right foot — hard acceleration is the single biggest variable.

None of that means anything is wrong with your car. It means the sticker was never your number.

MPG lies about savings

Here’s the counterintuitive part. MPG is an inverse measure, so equal jumps don’t save equal fuel:

Going from 10 to 20 mpg saves more gas than going from 20 to 40.

Over 12,000 miles, 10→20 mpg saves 600 gallons; 20→40 saves only 300. That’s why this calculator also shows gallons per 100 miles and L/100km — measures that scale with what you actually spend. When you’re comparing two thirsty vehicles, the cost-per-mile line tells the truth that the MPG number hides.

Fuel is only one line

The cost-per-mile figure here is fuel only — gas price ÷ MPG. The full cost of a mile also carries depreciation, insurance, maintenance, and fees, which usually dwarf the fuel. Use your measured MPG as the input to the bigger pictures:

The one-paragraph version

The EPA sticker is a lab estimate; your real MPG is miles ÷ gallons off your own tank, and it’s usually 10-25% lower. Measure it across a full tank (or average a few), then read cost per mile and gallons-per-100-miles rather than fixating on MPG, because MPG understates the savings from improving a thirsty vehicle. Run yours with the MPG calculator.

AutoMath is an educational tool. The numbers depend entirely on the miles, gallons, and price you enter.

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