In addition to the testing methods used to determine the EPA's ratings, a host of other physical and personal factors contribute to the differences between a vehicle's rated and realized energy consumption. For starters, cars and trucks used for evaluation in the EPA's tests are broken in and are in top mechanical shape. New vehicles don't usually attain their top mileage until they're driven about 3,000 to 5,000 miles, and ill-maintained vehicles will consume more gas than those that are in perfect condition. Even relatively minor upkeep factors like having incorrect air pressure in the tires can affect your vehicle's fuel economy.
Depending on where you live, the particular blend of gasoline sold in your area at a given time of the year may have more or less energy content, which in turn results in better or worse fuel economy. What's more, the EPA claims that even small differences in manufacturing and assembling can cause minor disparities in fuel economy from one otherwise alike model to another.
Also, the cars and trucks subjected to fuel economy testing are "driven" without a full complement of passengers, cargo, and options aboard -- all else being equal, the heavier a vehicle is, the more fuel an engine will need to burn in order to reach and maintain a set speed. Similarly, the vehicles are tested without the air conditioning and other electrical accessories in use, which also tends to put a greater load on the engine, and thus impacts the vehicle's fuel economy.
Other physical factors like trip length, traffic conditions, terrain, temperature, and the weather all affect your mileage. Likewise, installing exterior accessories like roof racks and cargo carriers that hamper a vehicle's aerodynamics will take their toll at the pump -- the more aerodynamic "drag" that's placed on a vehicle, the more energy it takes to run it, especially at highway speeds. Lead-footed acceleration, heavy braking, high-speed driving, excessive idling, towing, and engaging four-wheel drive will also drain your vehicle's gas tank at a higher-than-average rate. The EPA estimates that jack-rabbit starts and sudden stops alone reduce a car or truck's fuel economy by as much as 33 percent at highway speeds and five percent in the city.
An Imperfect, Yet Useful System
While the EPA's fuel-economy estimates may not be a completely accurate prediction of the kind of mileage you'll register during your daily commute, it's still valid as a source of comparison when you shop for a new vehicle. In addition to city and highway mileage estimates, a new vehicle's price sticker will show the fuel-economy range most drivers can expect to actually achieve with that particular model, the annual estimated fuel cost (based on 15,000 miles per year and a predetermined, though not mentioned, cost per gallon of gas), and the fuel-economy range for other models in its size class. The EPA's estimates for all vehicles can be found in a master list posted at www.fueleconomy.gov, and a printed version can be ordered via the website as well.
At the least, by checking this list you can get a relative idea of how one particular car or truck measures up against others in its class, or how one type of vehicle fares, on average, in comparison with others. If, for example, you're comparing two vehicles and one is estimated to get a third better fuel economy than another, you can reasonably expect to pay a third more to keep the latter's gas tank filled, all else being equal. This is also valid for noting the relative efficiency among available engines in a given car or truck's model range.
Change on the Horizon
The E.PA. is currently developing new, more accurate ways of testing vehicle fuel economy. The organization's new testing methods will take into account real-world driving factors like aggressive acceleration, hot- and cold-weather driving conditions, and the use of air conditioning while driving, for example.
Taking these widespread driving habits and conditions into account when determining fuel-economy averages should better align the E.PA.'s estimates with consumer's figures. Model-year 2008 vehicles will be the first to see the new E.P.A. fuel-economy numbers, which experts estimate will be about 12-percent lower than the current figures.
With demand for fossil fuel continuing to grow worldwide, and pump taxes remaining an increasingly popular source for state and local revenue, it's not likely that the cost of a gallon of gasoline will fall below the $2.00 mark any time soon, if ever. Some industry observers predict it might rise to as high as $4.00 or more a gallon in the coming years. Thus it behooves your pocketbook to heed the EPA's ratings, however suspect they might be, when shopping for a new vehicle to help you find a model in your size, type, and price range that will deliver the best possible fuel economy.