Fuel economy is the most honest health check a car has. A sudden drop can point to underinflated tires, a dragging brake, a failing sensor or simply a change in how and where you drive. To notice it, you need a real number, not a guess.
The full-tank method
- Fill the tank until the pump clicks off. Do not top up past the click.
- Note the odometer reading, or reset a trip meter.
- Drive normally until you need fuel again.
- Fill up the same way, to the first click, and note how many gallons it took.
- Divide the miles driven by the gallons you just put in.
Example: you drove 320 miles and the second fill took 11.2 gallons. 320 ÷ 11.2 = 28.6 mpg. You can do the same sum in our fuel economy calculator, which also converts to L/100 km and works out the cost.
Why it works: filling to the same point twice means the gallons you add the second time are exactly what the car used in between. That is why the first fill only sets the starting point and does not count.
One tank is not enough
Pumps do not all click off at exactly the same level, and one tank can be all highway while the next is all city. A single measurement can easily be a few percent off. Average three or more tanks, or better, track it continuously so each new fill-up refines the figure. Using the same station and pump helps when you can.
What about partial fills?
You do not have to fill to the top every time. Log every fill, partial or full, with gallons and mileage. When you next fill to the top, add up all the gallons since the previous full tank and divide the miles covered by that total. The answer is just as accurate, it simply covers a longer stretch.
How accurate is the dashboard?
Better than it used to be, but not perfect. In AAA’s 2021 lab tests, in-dash fuel economy displays were off by 2.3% on average compared with measured consumption, and the worst one overstated it by 6.4%, about 2.2 mpg. The error also moved around depending on speed and driving style. The dashboard is a good guide to how your driving affects consumption in the moment. For the real long-run figure, count the gallons.
From mpg to cost per mile
Divide the price per gallon by your mpg. At $3.40 a gallon and 28.6 mpg, each mile costs about 12 cents in fuel. That number makes comparisons easy: it tells you what a commute, a road trip or a second car really costs to run, and you can compare it directly with what an electric car costs per mile, covered in our guide on EV charging cost.
Converting to L/100 km
Outside the US, consumption is usually given in liters per 100 kilometers, where lower is better. To convert, divide 235.2 by the mpg (US gallons). 28.6 mpg is about 8.2 L/100 km. The calculator does this both ways.
What moves the number
- Speed. Drag rises steeply above about 50 to 60 mph.
- Cold weather and short trips. A cold engine runs rich until it warms up.
- Tire pressure. Low tires add rolling resistance.
- Load and roof boxes. Extra weight and drag both cost fuel.
- Air conditioning and idling.
- A problem with the car. If mpg drops noticeably with no change in driving, have it checked.
Track it automatically
In Badger, every fill-up goes into the fuel log with gallons, price and mileage. It measures real fuel economy between full tanks, keeps a running average, and adds the cost to your expenses. Each fill-up also updates the car’s mileage, which keeps the maintenance schedule accurate without extra typing.
Common questions
Is it bad to fill only partway?
Not for the car. For measuring mpg, just log the partial fill and measure from one full tank to the next.
Why is my mpg lower than the EPA sticker?
EPA figures come from standardized lab tests. Real driving with more speed, cold starts, short trips and climate control often comes in lower, sometimes noticeably.
What is a good mpg?
It depends on the car. The useful comparison is with your own car over time: a steady figure is good news, and a sudden drop is worth investigating.
