A version of the boilerplate phrase, “Your mileage may vary,” is typically embedded in new-car ads whenever fuel economy is cited, and with good reason. With gas prices continuing to take a significant bite out of a family’s budget, a common complaint among disgruntled consumers is that their vehicles’ realized mileage often falls short of the automakers’ estimates.
The latest wave of small turbocharged engines in particular have tended to draw ire for what independent testers have found to be overly optimistic fuel economy ratings, and a few automakers have even been cited by the federal government in recent years for using what were later found to be overstated MPG figures.
Much of the discrepancies can be traced to the fact that cars are tested, not on the open road, but in a laboratory under controlled conditions. In fact, the vehicles don’t reach the pavement at all. Rather, a car’s fuel economy is measured in a laboratory on a machine called a dynamometer (it’s like a treadmill for cars) under controlled conditions mandated by federal law.
While the Environmental Protection Agency, which oversees fuel economy ratings in the U.S., updated the testing system in 2008 to afford more accurate results, they can still miss the mark. Fortunately, more accurate MPG projections could finally be on the way. The EPA is reportedly considering regulations that would mandate automakers use on-the-road data to validate their advertised mileage claims.
As it stands, a professional driver runs a vehicle being tested in a laboratory through five separate standardized “driving” schedules, one each to simulate city traffic, highway cruising, driving at higher speeds, operating a car with the air conditioning on and in stop-and-go driving with lower ambient temperatures. A hose is connected to the vehicle’s tailpipe that collects the engine’s exhaust, and the amount of carbon present in such emissions is used to calculate the volume of fuel used. Automakers actually do their own evaluations and submit the results to the EPA, which subsequently reviews and confirms the data. While some automakers already make use of on-the-road data to verify results gleaned from stationary tests and computer-simulated estimates, the EPA’s proposal would both require and standardize that procedure. For example, the agency wants manufacturers to measure air-resistance and rolling friction on a test track and not just a computer model. Such data is said to dramatically affect a given vehicle’s fuel economy rating, and can be especially important in correctly evaluating high-mileage models like hybrids.
But 100 percent accuracy in this regard will likely still be a moving target. That’s because a host of other real-world variables can affect a given vehicle’s energy consumption, sometimes significantly. For starters, even relatively minor upkeep factors like having incorrect air pressure in the tires can cost an owner a few MPG. Also, added options, the number of passengers aboard and the weight of cargo in the trunk can affect a model’s mileage; 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.
Other physical factors like the particular blend of gasoline sold in a particular area at a given time of the year, trip length, altitude, ambient temperature, and traffic, road and weather conditions all affect a car’s mileage. Certain exterior accessories like roof racks can increase a vehicle’s aerodynamic “drag,” and will in turn decrease a vehicle’s mileage, especially at highway speeds.
Heavy acceleration and braking, high-speed driving, excessive idling, towing, and engaging four- or allwheel drive also tend drain a vehicle’s gas tank at a higher-than-average rate.
In other words (to paraphrase the poet Robert Burns), despite the bestlaid schemes of mice and men, your mileage may still vary.
© CTW Features