How to track your car's fuel use and cost per mile

A fuel log turns gas receipts into two numbers: miles per gallon and fuel cost per mile. At each fill-up you write down three things, the odometer reading, the gallons and the total you paid. A notes app, a spreadsheet or the free My MPG tool at fueleconomy.gov all work.

What you need

• A notes app, a spreadsheet or the My MPG tool
• The pump receipt, or the pump's display
• The car's odometer, or a trip meter you reset at each fill-up

Steps

1. 1

   Fill the tank and record the odometer

   Fill the tank all the way and write down the odometer reading. This first entry is the starting point, so it has no miles per gallon yet.

   1 min

2. 2

   Record each fill-up after that

   Each time, fill the tank all the way and write down the odometer reading, the gallons and the total cost. Use the total, not the price per gallon.

   1 min

3. 3

   Work out the miles driven

   Subtract the last odometer reading from this one. If you use a trip meter, read it and reset it.

   1 min

4. 4

   Work out miles per gallon

   Divide the miles driven by the gallons it took to refill the tank.

   1 min

5. 5

   Work out the fuel cost per mile

   Divide the total cost by the miles driven. For a month, add up the month's fill-ups and divide by the month's miles.

   1 min

6. 6

   Check for a missed fill-up

   A missing fill-up throws off the numbers. If you skipped one, mark the gap and start again from the next full tank.

   1 min

7. 7

   Check the numbers look right

   Each line has an odometer reading, gallons and a total, and the miles per gallon is close to the line before it.

   1 min

Reading the log

When miles per gallon drops from one month to the next, look for the cause. Underinflated tires lower mileage by about 0.2 percent for every 1 psi drop in average pressure, so check the pressure first. The wrong grade of motor oil can cost 1 to 2 percent. If the check engine light is on, have a mechanic look at the car, since some problems also waste fuel.

An annotated example