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1.
A person who exceeds the Environmental Protection Agency's(EPA) fuel economy ratings for automobiles.

For example, if a Honda Accord has an EPA issued miles per gallon rating of 26 miles per gallon in the city and 33 MPG on the highway, then a hypermiler will manage to get at least 35 MPG in the city and around 43 MPG on the highway.

In order to become a successful hypermiler, one must sacrifice a little bit of speed and time, and perhaps comfort. For example, by not using your air conditioner in the car you can save a good bit of fuel. Also, another way to save gas is to avoid jackrabbit starts and abrupt brakes. Another method is to turn off your car when waiting at a long red-light. Another way is to slow down while climbing up a hill and to get off the gas while going down a hill, or if you have a standard transmission, put it in neutral and even possibly turn it off. There are numerous other ways(including some that may be illegal in your state, such as drafting behind 18-wheelers on the interstate), but perhaps the most simple way is to make sure your car's tires are pumped to at least the recommended psi rating(preferably about 7 or 8 psi more), and to keep your car maintained well.

There are expert hypermilers out there, such as Wayne Gerdes, who have achieved over 100 miles per gallon using hybrid vehicles such as the Honda Insight. He has even achieved the amazing feat of getting 59 miles per gallon with a regular Honda Accord.

So what's the point of hypermiling? Basically, you save gas, and hence you save wampum. If you drive daily you can easily save an extra 200 bucks a year using these techniques - that's enough to get yourself a cool gadget or eat out at some swank places a few more times. You also drive in a more calm manner, and you even help out the environment. You can be a hypermiler with any car. Hypermiling is about the driver, not the car.
Adam: "Damn, my Toyota Camry is supposed to be getting 30 miles per gallon, but instead I'm getting about 22 miles per gallon. How can you explain that? I hate how those car dealers lie."

Brad: "Look at your car douchebag. One of your tires looks like it's almost flat, you have crappy alignment, your trunk is filled with a bunch of junk, you drive like a madman, and by the way - when's the last time you got an oil change?"

Adam: "Oil change? Uhhhh... errr... what does that have to do with anything?"

Brad: "A lot. Listen up, seriously. Take care of your car, and if you want to get much better fuel economy, I got one word for you pal: hypermiler."

Adam: "What? Did you say hypermiler? I never heard of that, are you making this up?"

Brad: "OK, I got one more word for you: Wikipedia. Now go fix up your baby and read up. Then we'll talk."
by Adel7 August 21, 2007
 
2.
A Hypermiler is someone who drives 37.2 MPH on a interstate so they can save $0.75 cents on gas over the course of a week. They have no regard for anyone else or their time.

Hypermilers present a clear danger to to traffic by creating an unanticipated obstruction by simply driving well below the posted speed limit and expected speeds for a given road.
That hypermiler just caused an accident with a bus full of nuns to save fifteen cents on his daily commute!
by ABadCaffeineTrip June 24, 2008