A process so simple and fast, even I can do it.
The next time you fill up your gas tank, reset the mileage counter on your car.
The time after that, make sure to fill up the tank all the way and then get the receipt for your gas. Note the number of gallons it took to fill up your car.
Divide number of miles on your mileage counter by the number of gallons you just filled up.
This is your gas mileage!
Example: Today, I filled up the tank on my 15 year old Geo Prizm, which is still kickin' it old school. Sure, it has a few (large) dents here and there. Sure, the overhead light doesn't work. Sure, sometimes the radio goes out when I drive over a speed bump. But....
The mileage counter read 201 miles today. It took 7 gallons to fill up my tank. This means I have an effective gas mileage of 28.7 mpg. Not bad for a 15 year old car.
I regularly check my gas mileage to make sure that the Prizm doesn't have a problem. I just have a habit of resetting my mileage every time I get gas, which makes this a very easy and quick process. Normally, the Prizm's mileage runs about 29-30 mpg, so I'm still in a tolerable range. If it fell too much, I would need to have the tires aired up, see if the air filters or spark plugs needed to be replaced, make sure to drive efficiently, and have the car checked out. Here's some tips for optimizing your mpg.