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Facts and Data - What MPG Can Tell You

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Consumers report that one of the most confusing and annoying things about purchasing cars are the often overly optimistic Miles Per Gallon (MPG) figures quoted on the window sticker. These numbers were originally calculated by the EPA to determine a car's emissions but were quickly used by people to compare model A's fuel efficiency against model B's gasoline costs. Almost immediately, buyers complained that the reported numbers were not matching real world driving experiences.

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Back when the first "gas crunch" happened in the 1970's, car manufacturers became so fixated on getting higher numbers that, legend has it, unscrupulous auto makers would go so far as to sneak mechanics into the EPA test facility and do a tune up on their company's cars to boost the MPG rating a few points (and improve emissions results too). Unfortunately, these EPA estimates seldom matched the real-world mileage experienced by car owners. While the test process was never intended to give the buyer an exact gas mileage number, people assumed that's what it was, so naturally, disappointment was common. In response, the EPA has, over the years, modified the testing process and even, recently, dropped the their published estimates a few percentage points in an attempt to make their numbers match drivers' real world experience. Today, most EPA numbers are getting close to real world milages but since the tests are not conducted on the road - rather they re run in a laboratory setting - things like aerodynamics are not factored in. Here's how you can check your car's real-world fuel mileage for yourself:

You may be surprised (unless you are a math teacher) to learn that many people don't know how to calculate their car's fuel mileage. While the process is simple, it requires the owner to do a few simple but important things to accurately measure what his or her vehicle's MPGs are. Here are the steps:

  • When you get gas, request a full tank - that's a critical starting point
  • While sitting at the station, reset the trip odometer (or write your car's miles driven on a piece of paper)
  • Drive normally - do not try to be extra frugal (more on how to do that later)
  • When the gas gauge reads 1/4 tank or lower, go and get gas - again, requesting a fill-up
  • Record the number of gallons it took to fill the tank on a piece of paper
  • Record the miles driven on the same piece of paper - from the trip odometer or by subtracting your car's mileage when you last filled from the odometer reading at this fill-up.
  • Divide the miles driven by the number of gallons it took to fill your tank - that's YOUR car's MPGs
  • EXAMPLE: Your car traveled 275 miles since its last fill up and it took 13.3 gallons to fill your tank
  • Your car's mileage for that last tank was (275 / 13.3) = 20.68 miles per gallon or about 21 MPG

If you regularly check your mileage, this information can provide you with what experts call "actionable intelligence". For example, if your mileage drops by 10% to 15% (or more), your car may be developing a problem. This could be: low tire pressures (low pressures increase friction and lower fuel efficiency), a dragging brake pad / sticking caliper (a serious safety problem), clogged, dirty fuel injector(s), clogged or dirty air filter, wrong oil viscosity in the engine or very dirty oil, clogged catalytic converter or a collapsed tail pipe, or your car is in need of a full tune up. There is no need to panic if your mileage drops after being stuck in traffic - idling in bumper to bumper for an extended period of time. It should go back up the next time you get gas. If it doesn't, then you can investigate the reasons for the drop at your service station's mechanic. Also, cold weather will lower your gas mileage because your car has a technology called a "cold start valve". This device increases the amount of fuel (and raises the idle speed) your car uses during warm-up to ensure that it doesn't stall or run improperly. If you do a lot of short runs in the colder weather, your car will spend more time running a "richer"gas mixture at higher idle speeds, thereby lowering your mileage.

Conversely, your fuel mileage could improve significantly (2 to 5 MPG) if you go on a long trip with a lot time spent traveling highway miles. Stopping and starting cuts mileage so cruising at legal highway speeds usually bumps up your MPG number.

Some common sense tips to improve your car's fuel milage:

  • Empty the trunk - carrying extra weight hurts your mileage
  • Use a good quality fuel - low grade fuels lack the detergent additives that keep fuel injectors clean - dirty fuel injectors mean poor mileage
  • Keep tire pressures at your manufacturer's recommended levels and - very important - be sure to check them when the weather turns cold. Look on the door post on the driver's side for this important information
  • Use the right viscosity oil - check your owner's manual for what you should be using and always use a top quality oil and filter.
  • Drive sensibly - fast starts and speeds above 75 MPH significantly cut fuel mileage
  • Follow your manufacturer's recommended tune-up schedule - spark plugs, fresh oil and clean air filters are important

Once you know your car's MPG, you can even calculate whether it makes sense to drive a few miles to fill up at the station with gas that's a few pennies cheaper (usually it's not) and even if buying that super efficient high MPG Hybrid makes good economic sense (also, usually not).

More on that in another article.

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