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Fuel economy (or mileage) is the average distance a vehicle can travel on a specific volume of gasoline. Common units to express fuel economy are:
  • Miles per Gallon (MPG) ; the number of miles a car can travel on a gallon of gasoline. A high MPG indicates a more efficient performance of the vehicle.
  • Kilometers per Liter (km/L); Sometimes (confusingly) expressed as 1 : x, where x is the number of kilometers travelled. This rating is the metric equivalent of the MPG quantization that is common in the U.S.
  • Liters per 100 Kilometers (L/100 Km). Another popular measure for fuel economy. In this case, a higher quantity indicates a lower fuel economy.

Converting the various units is easy: 1 MPG = 0.425 Km/L, 1 Km/L = 2.35 MPG. See table below.

  MPG    km/L    L/100 km
  5.0     2.1      47.0
 10.0     4.3      23.5
 20.0     8.5      11.8
 30.0    12.8       7.8
 40.0    17.0       5.9
 50.0    21.3       4.7
 60.0    25.5       3.9
 70.0    29.8       3.4
 80.0    34.0       2.9
 90.0    38.3       2.6
100.0    42.5       2.4

Government agencies, such as the U.S. Environmental Protection Agency (EPA), as well as several automobile manufacturers often test vehicles for their fuel economy. These are standardized tests that allow for a direct comparison between different brands and models of cars. The vehicles are tested in a laboratory by professional drivers, using a test setup similar to a treadmill.

The EPA annually reports fuel economy estimates for most cars sold in the U.S.; This data consists of two different estimates of the fuel economy; a city, and a highway estimate.

The test used to determine the city fuel economy estimate simulates a 7.5-mile, stop-and-go trip with an average speed of 20 miles per hour (mph). The trip takes 23 minutes and has 18 stops. About 18 percent of the time is spent idling, as in waiting at traffic lights or in rush hour traffic. Two kinds of engine starts are used: the cold start, which is similar to starting a car in the morning after it has been parked all night; and the hot start, similar to restarting a vehicle after it has been warmed up, driven, and stopped for a short time.

The test to determine the highway fuel economy estimate represents a mixture of "non-city" driving. Segments corresponding to different kinds of rural roads and interstate highways are included. The test simulates a 10-mile trip and averages 48 mph. The test is run from a hot start and has little idling time and no stops (except at the end of the test).

Since the fuel economy estimates reported by the EPA are obtained in a laboratory, the results are generally adjusted for actual driving conditions on the road; i.e. the city estimate is lowered by 10%, and the highway estimate by 22%.

Go to http://www.fueleconomy.gov for more info, and to find fuel economy data of various automobiles.

I commute a very long distance to work each day. From my driveway to the parking lot at work is about seventy-five miles. I’ve been doing this for about two years. Some of my friend think I’m crazy, and ask what I do to pass the time on the drive. For the longest time I made this daily pilgrimage to work solo, and my 1992 GMC Sonoma was not too comforting for such a trip. The tape deck had been burned out for years, and the radio’s reception wasn’t good enough to hold the same station the whole way. All my pre-programmed settings were divided to “at-home” and “in-Harrisburg”. What I found myself doing to keep my sanity was watching and trying to advance my fuel mileage.

Watching your fuel mileage can tell you a lot about your driving habits and the current state of your vehicle. If I drove the speed limit and kept the engine in good order (changing oil and filters on a regular basis) I could get about three hundred and seventy miles per gas tank. The math is simple, but breaks down as thus:

370 miles divided by an 18-gallon gas tank equals 20.5 miles per gallon.

This is not good for a commuter vehicle, but not bad for a pickup truck. I know the location of every gas station along my trip, so I’d run the truck until I was almost out of gas on a regular basis. I read once that this is good, as water condensation in the gas tank can build up and change your fuel mileage. If I let the oil change come and go, which is to say if I let it stretch from the recommended three thousand miles to five, I’d only be able to get about three hundred miles per tank, or 16.6 miles per gallon. This, in my horrible state of financial affairs, is unacceptable and prompted me to stay true to upkeep on my vehicle.

I also played with urban legends about gas economy. I have heard people say that if you put the tailgate down on a pickup it becomes more streamline and therefor passes through the air more effortlessly. This makes sense to me, but my father tells me that this is a misconception, and that the designers of the truck counted on the tailgate to add rear downforce on the back tires. Without that downforce the rear tires would not be able to deliver the force needed to propel the truck as efficiently. I pondered this conundrum, and decided to test it myself. This resulted in three hundred and eighty-five miles per tank, or 21.39 miles per gallon.

All of this testing changed early this year when I finally retired my Sonoma for a 1996 Nissan Maxima. I found that the car got much better gas mileage. Roughly five hundred miles per tank. Here’s the new math:

500 miles divided by a 16-gallon gas tank equals 31.25 miles per gallon.

I was very happy with the obvious improvements, although since I found myself bored with my fuel mileage game. There were very few options now. Also, in the past six months my girlfriend has accompanied me on this daily trip. I find the conversation much more stimulating that running the math on the efficiency of my trip. Sometimes I do wonder if I could get more out of my car. I wonder if I can squeeze the fuel mileage from it like someone may squeeze juice from a lemon. This is infrequent though, and passes like the roadside mile markers.

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