— Ken Mickley, Allentown
A: Interesting observation, Ken, and it came my way by virtue of its recent publication as a letter to the editor. Introduced in the late 1970s in the wake of a slowdown in world oil-supply traffic and a corresponding spike in gas prices, the U.S. Environmental Protection Agency's automobile gas-mileage estimates were not intended to serve as precise predictors of real-world mileage.
The phrase "your actual mileage will vary" appears on today's EPA stickers, and that concept was trailer-hitched to the estimates at the starting line. Too many variables come into play to predict mileage with great accuracy. Instead, the estimates are intended to help consumers make mileage comparisons between different models.
Motorists who calculate the gas mileage repeatedly on the same vehicle for a number of consecutive fill-ups will find that it varies at least somewhat even from one tankful to the next. Mileage is derived by fully filling the tank each time, and dividing the miles traveled by the number of gallons used. If you cover 300 miles between fill-ups, and it takes 12 gallons to fill the tank, you got 25 mpg on that tankful.
I checked the mileage on my four-cylinder 1993 Honda Accord for about a year after I bought the car, new, in spring of that year. The record of those calculations has long vanished, but I remember the mileage varying from the mid-20s to about 34 mpg, depending largely on the measure of steady-speed "highway" driving versus stop-and-go "city" conditions I encountered. The average was in the high 20s.
That car still was getting pretty good mileage when I totaled it last September in an accident that was, in my own judgment, my fault. The shortcut version: I was going too fast for conditions (rain) in the right lane of one-way Chew Street in Allentown when a car pulled out from a side street ahead of me. When I steered left and hit the brakes, the car slid rather than stopping — which it finally did upon hitting a truck parked at curbside. No injuries, and the cars were drivable, so police declined to respond. My insurance paid for all repairs, but without "collision" coverage on my 18-year-old, 180,000-mile heap, it was history. (I can almost hear reader Barbara Rowley murmuring, "I told you so." As I lambasted Allentown officials last year for reducing the speed limit on Parkway Boulevard, the retired college professor lectured me about driving too fast.)
Forced by circumstance to come up with another car, I bought a Fiat 500. In the late '70s I had two Fiat 128s, fun little sedans that handled like sports cars. Unfortunately, Fiats of that era richly deserved their reliability status as the butt of endless jokes, and the Italian car-maker took the exit lane from the U.S. market for three decades until last year's debut of an Americanized version of its minuscule 500. It's not the most practical car — a half-foot shorter than a Mini Cooper, it's rated for four passengers but offers only a rear-seat facsimile — but it's a zippy little thing that's a blast to drive (so long as you're careful in the rain), and Fiat claims world-class quality (we shall see; no problems so far).
The car gets decent, if not impressive mileage, considering its featherweight 2,500-pound stature. The EPA estimates are 30 city, 38 highway and 33 combined. My calculations have ranged from 32.1 to 34.8 mpg, with a 3,300-mile average of 32.9, almost exactly the EPA figure for combined driving. I haven't yet taken the vehicle on a long highway trip.
You're right, though, Ken, not everyone's mileage will conform as closely with the government estimates. In addition to highway-versus-city conditions, factors including driving habits, vehicle condition, weather, topography, air conditioning and other accessories, and what the EPA terms "fuel characteristics" can affect mileage significantly.
The "fuel characteristics" include ethanol content. The government uses pure gasoline without ethanol, and "drives" the cars on dynamometers, not in real-world conditions. But those controls are imposed to make the estimates comparable, maintaining test conditions that are as similar as possible between different models. The dynamometer mimics various driving conditions by imposing different levels of resistance, but it applies the same conditions to each car — a feature road testing can't provide.
The pure-gas choice helps build a level road for testing as well: "While it is common for gasoline pumps to allow for up to 10 percent ethanol," the actual proportion varies, according to the EPA, as do fuel-blending requirements from state to state. A 10 percent blend should reduce mileage by 3 percent to 4 percent, which could account for a portion of your reduced mileage, Ken.
In response to concerns similar to yours — that actual mileage too often was significantly lower than the EPA estimates — the government tweaked the testing method beginning with the 2008 model year, in effect hitting the gas harder to better reflect the heavy-footed American driving style. That prompted mileage estimates generally to fall a bit from the 2007 figures, even for unchanged models.
As my Fiat experience suggests, the new estimates probably do reflect a little more closely the mileage most people achieve. But the modified testing didn't really change the substance of the program. The mileage estimates for most vehicles declined a bit for 2008, but the proportional change was pretty steady across the field. For example, if car A went from 25 to 23 mpg and car B from 32 to 30 mpg, there's no meaningful difference when comparing the two models under the new test method compared to the old.
But again, officials never suggested that individual drivers necessarily would achieve the mileage cited in the estimates. The disclaimer always applied: Actual mileage will vary. So you're right, a motorist should never buy a car on the assumption the mileage will meet or exceed the EPA estimates. But the estimates can be useful in helping choose one model over another. With gas prices expected to be traveling on a one-way street with a steady incline, good mileage will be an increasingly important factor for car buyers.
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