Drivers: Your Mileage Will Vary



2005 Honda Accord Hybrid by Marc K. Stengel (7/4/2005)
The numbers don’t add up – but it’s still a smart buy.

TCC Tip: Shopping for Hot Cars by Eric Peters (6/13/2005)
Hard-to-get cars can be had — here’s how to get them.It’s a common complaint among new-vehicle owners, and it’s an especially ripe one given the cost of a gallon of gasoline these days. Drivers are complaining that their real-world fuel economy doesn’t match up with the official estimate given by the Environmental Protection Agency (EPA). These figures are regularly included in published and Internet car guides, are posted on the window stickers of nearly every car and truck sold in theU.S., and are highly touted in automakers’ advertising for all but the most-gas-swilling models.


So why doesn’t the car or truck you drive to and from work every day get the same fuel economy the government, the manufacturer, and us trusted legions of automotive journalists seem to promise it will?


The answer is as simple as it is ultimately complex. New vehicles, as tested for their energy consumption, are never actually driven anywhere, much less to and from work, and their fuel economy ratings are not ultimately based on how much fuel they consume.


Ratings versus reality

It would seem logical to determine a vehicle’s fuel economy by just filling up the tank, driving it for a set number of city and highway miles, refilling the tank, and dividing the number of miles driven by the number of gallons consumed, right?


While this is how any motorist might keep track of his or her mileage on a casual basis, it’s hardly the kind of scientific testing upon which professional engineers would stake their reputations. In fact, a car or truck’s fuel economy is measured under rigidly controlled circumstances in a laboratory using a standardized test that’s mandated by federal law. Automakers actually do their own testing and submit the results to the EPA, which reviews the data and confirms about 10- to 15-percent of the ratings itself at the National Vehicles and Fuel Emissions Laboratory.


Each model is tested on what’s called a dynamometer, which is like a treadmill for cars. While the engine and transmission drive the wheels, the vehicle never actually moves — just the rollers upon which the wheels are placed. A professional driver runs the vehicle through two standardized driving schedules, one each to simulate city and highway motoring, and ensures he or she is maintaining the mandated pace via a computer display.


The “city” program is designed to replicate an urban rush-hour driving experience in which the vehicle is started with the engine cold and is driven in stop-and-go traffic with frequent idling. The car or truck is driven for 11 miles and makes 23 stops over the course of 31 minutes, with an average speed of 20 mph and a top speed of 56 mph. The “highway” program, on the other hand, is created to emulate rural and interstate freeway driving with a warmed-up engine, making no stops (both of which ensure maximum fuel economy). The vehicle is driven for 10 miles over a period of 12.5 minutes with an average speed of 48 mph and a top speed of 60 mph. With both tests the vehicles’ air conditioning and other accessories are turned off.


Sound like your daily commute? Thought not.


Throughout the test a hose is connected to the vehicle’s tailpipe and collects the engine’s exhaust. It’s the amount of carbon that’s present in what’s spewed from the exhaust system that’s measured to calculate the amount of fuel burned. The EPA claims this is more accurate than using a fuel gauge to physically measure the amount of gasoline that’s being burned. Still, the final test figures are adjusted downward, by 10 percent for city driving and 22 percent in highway mileage, to help reflect the differences between what happens in a lab and out on an actual road.


Hybrid hype?


As has been well publicized, the discrepancy between posted and real-world fuel economy can be even more pronounced for owners of gas-electric hybrid vehicles. Most sources we surveyed feel the EPA’s ratings for hybrid vehicles tend to be overstated by a factor of at least 20 percent. This discrepancy can be wider yet if a motorist drives primarily on the highway, where hybrids tend to be less efficient than in stop-and-go city driving conditions (during which the electric motor shoulders more of the effort).


In fact, the results of ongoing operating tests conducted by the EPA of a dozen hybrid cars in its own fleet significantly contradict their posted fuel economy ratings. When last we checked, the best the EPA’s fleet could muster was a cumulative average of 37.7 mpg for the Civic, 45.7 mpg for the Insight, and 44.8 mpg for the current-generation Prius. While this is certainly admirable fuel economy, it’s still far below the cars’ official EPA estimates that run as high as 51, 66, and 60 mpg, respectively for the model years tested.


Why do such discrepancies tend to be more pronounced for hybrids? Basing fuel economy upon the amount of carbon exhaust that’s emitted by a vehicle’s tailpipe automatically favors gas/electric-powered vehicles. Since some of a hybrid’s power comes from an electric motor that automatically produces zero emissions, these figures tend to skew higher than simple miles-driven/gallons consumed computations would otherwise indicate.


Other factors


And it’s more than just the “laboratory versus real world” explanation that’s to blame for the inconsistency in fuel economy ratings. A host of physical and personal factors contribute to a vehicle’s 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 broken in, which occurs at about 3000 to 5000 miles of (fairly gentle) driving, and ill-maintained vehicles will consume more gas than those that are in perfect condition. Depending on where you live, the particular blend of gasoline sold in your area may have more or less energy content, which in turn results in better or worse fuel economy. What’s more, even small differences in manufacturing and assembling can cause minor disparities in fuel economy from one otherwise alike vehicle to another.


Also, the cars and trucks are tested 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. Likewise, 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. (Note, however, that driving at highway speeds with the A/C off and the windows open will not save gasoline. The open windows will cause enough wind resistance to the vehicle that you could actually consume more gas than you would by driving with the windows closed and the A/C on.)


Other physical factors like trip length, traffic conditions, terrain, temperature, and the weather all affect your mileage, as will attaching accessories to your vehicle like roof racks and cargo carriers that hamper a vehicle’s inherent aerodynamics. Lead-footed acceleration, heavy braking, high-speed driving, excessive idling, towing, and engaging four-wheel-drive also negatively impact your vehicle’s mileage. The EPA estimates that jack-rabbit starts and sudden stops alone can reduce a car or truck’s fuel economy by as much as 33 percent.


The price sticker affixed to every new-vehicle window is required to carry the EPA’s information about the car or truck’s fuel economy (except for those having gross vehicle weight ratings over 8500 pounds, like heavy-duty pickups and big SUVs like the HUMMER H1 and H2). In addition to city and highway mileage estimates, the 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.


While this is far from being a perfect rating system, consulting the EPA’s mileage figures when shopping for a car or truck remains a good idea. At the least, you can get a relative idea of how one particular car or truck compares to others in its class, or how one type of vehicle fares, on average, in comparison with another. If, for example, one car is estimated to get 30 mpg while another is rated at only 15 mpg, you can reasonably expect to pay half as much per year to keep the tank filled by selecting the higher-mileage model, all else being equal. This is also a valid for comparing the relative efficiency of, say, a car or truck’s optional V-8 engine versus the standard V-6 on a given model.


Still, as they say, your mileage may vary.


The Car Connection Daily Headlines
I agree to receive emails from the site. I can withdraw my consent at any time by unsubscribing.
Thank you! Please check your email for confirmation.