how do i figure it out. do i take the miles divided by gallons.

You got it. mpg is miles/gallons. Basically just reset your trip odometer with that little button when you fill up. Then the next time you fill up, just divide that trip odo reading by the gallons on the pump and boom....you've got miles per gallon (mpg). I keep a cheap calculator in the car for quick checks.

 

A few notes: when you fill up, you want to try to fill the same way you did before; i.e. if you topped off (pumped past auto shutoff) you want to try to do it the same way on the next fillup.

 

Also, to get repeatable readings, you want to fill up at the same station using the same octane gas. Only after a good number of tank fulls can you reliably compare between fuel stations. Also, ambient temp, a/c use, foot in the pedal, and other factors can impact mpg.

Yeah. Go to the pump and fill 'er to the top. Divide the amount of miles on the trip (assuming you reset it when you last filled up) by the number of gallons you put in. Pretty simple.