It's a Gas!
How To Calculate Miles Per Gallon – Using Elementary School Math
Written by: Larry Coffey
If you bought your car from a dealer, there was a sticker on the side giving you the gas mileage for highway and for city driving. That's great, except that your commute is probably a little of both, and resembles nothing like the test track used to measure gas mileage. Furthermore, you probably aren't a trained test track driver, privy to inside knowledge that makes you an expert at squeezing the last drop of gas from a new automobile.
No, you're probably more like me, sometimes making a jackrabbit start, just to get ahead of the annoying guy singing to his radio in the car next to you at the stoplight. Or maybe you break harder than normal because while you were trying to find the station he was listening to on your own dial – because despite his singing it was a cool tune – that you didn't see the bus pull out in front of you.
Though a bit silly, I use those points to illustrate that our own driving habits and roads are very different from how the gas mileage on your new car sticker was computed. So, how do you figure your real world gas mileage?
Calculate Miles Driven on a Tank of Gas
The simplest way, the one I describe here, requires two fillups. You first take an odometer reading on the initial fillup, then drive as you normally would until it's time to get gas again, and then note the amount of gas it takes to fill up the tank again (in gallons, not dollars), and take a second odometer reading before you leave the gas station.
On paper it goes something like this. First calculate the number of miles you've driven by subtracting the first odometer reading from the second:
Second odometer reading:  _ _ _ _ _ _ _ _ 
– First odometer reading:  _ _ _ _ _ _ _ _ 
_______________ 

Total miles driven:  _ _ _ _ _ _ _ _ 
Subtract the first odometer reading from the second odometer reading to get the total miles driven.
Calculate Miles Per Gallon
Now you take the total miles driven and divide that by the amount of gas (in gallons) you just bought.
Total miles driven: _ _ _ _ _ _ _ _ 

=  Miles per gallon: _ _ _ _ _ _ _ _ 

Gallons purchased: _ _ _ _ _ _ _ _ 
Divide the miles driven by the number of gallons to get your car's gas mileage.
How To Calculate Miles Per Gallon – Using A Java Applet
Here's the simple method. Follow the steps below to gather the numbers, and then plug them into the Java applet below.
 Fill up your gas tank and write down the number on your odometer.
 Drive your car normally for a few days, or until it needs more gas.
 Fill up your gas tank and again write down the number on your odometer.
 Write down the amount of gas you put into your car (in gallons).
 Plug those numbers into the applet below and press Compute!.
And that's all there is to it! Happy motoring!
Photo Credits: Gas Can by Alisha Vargas