http://www.motherearthnews.com/green-transportation/check-gas-mileage.aspx

How long has it been since you last checked the real-world gas mileage of your car? Have you *ever* done it?

It can be all too easy to assume your car gets 30-something mpg — or whatever the sticker said when you bought the car — and never realize that its actual gas mileage has declined over the years and now is far off what you expect. But it's important to keep tabs on your car or truck's mpg.

First and foremost, "knowing is half the battle," as they used to say in the old G.I. Joe cartoon PSAs from my childhood. In other words, the first step to increasing your car's gas mileage (and saving money) is to know what it actually gets. Also, unless you're lucky enough to have a real-time mpg display in your car, there's no better way to better understand how driving habits influence gas mileage than to check the numbers. As in, on this tank I ran the air conditioning more than usual (you would see the mpg decline), or on this tank I drove the speed limit to work rather than speeding to make up lost time (you would see improved mpg). Last but not least, regularly checking your vehicle's mpg can spot maintenance issues before they become leave-you-stranded and wicked-expensive problems.

If you're a gas mileage geek like me, you'll calculate your car's mpg after every fill up. But even just checking mpg once a month or so will give you real numbers that will make it easy to adjust your driving habits so you save gas and money. Think of it as a personal challenge and you'll find it easier to get excited about the math — can you beat last month's personal best of 36 mpg? Can you beat the official EPA fuel economy estimate for your car?

And, to top it all off, you'll probably be surprised how easy it is to calculate gas mileage. Even a mathematically disinclined journalist like myself can do it. All it takes are two numbers and simple division. How to Calculate Gas Mileage will walk you through the steps.

For what it's worth, here's my system. I write down the number of miles on my trip meter on the credit card receipt from the gas station, which lists the number of gallons I bought. I also write down the car's overall mileage, just to help me track the car's mpg over time. I then plug all those numbers into a super-simple Excel spreadsheet, which does the simple division for me and automatically fills in the mpg for each tank. I can then track the car's mpg over time and see how it changes with the seasons, my driving habits, as I put off maintenance, after I got new tires, etc.

So, do you ever check your gas mileage? If so, how do you do it? If not, why not? Share your two cents by posting a comment below.

*John Rockhold is a green car enthusiast and Contributing Editor for *MOTHER EARTH NEWS*. Find him on *Google+.