Why US Mileage Ratings Are So Inaccurate

Why does a car rated for 47mpg fall so far short? The Houston Chronicle features an article on just why EPA gas estimates can be so different from real-world drivers' experience at the pump (or in looking at the dashboard display), in particular for hybrid cars. From the article: "A geometric average of the FTP-75 and HFET results (with city driving weighted at 55 percent and highway driving weighted at 45 percent) produces a vehicle's CAFE fuel economy, which is then incorporated into a manufacturer's corporate average. CAFE is measured using these tests to the present day. In fact, this methodology will be 50 years old when it's used to gauge compliance with the forthcoming 54.5-mpg CAFE requirements in 2025. That kind of continuity is admirable in baseball, but not in transportation. These tests are irrelevant to contemporary real-world driving. For example, the maximum acceleration on either test is 3.3 mph per second. At that rate, it takes more than 18 seconds to hit 60 mph. Even in the horsepower-deprived 1970s, most people were driving harder than that. And the 60-mph maximum speed on the highway test does not accord with the 75-mph truth of today's interstate traffic."

Comments

consistency more important

[mondovoja]

Whether those numbers represent a real world mix of driving accurately really doesn't matter all that much, since fuel economy for other driving styles strongly correlates with fuel economy for the conditions that are actually measured. Long term consistency, on the other hand, matters a great deal for car buyers and for evaluating progress on reducing emissions and consumption.

Re:consistency more important

[nabsltd]

I think that over a certain speed, fuel economy goes way down.

The smaller the engine, the more you will see this effect. This is because it has to work much harder (relatively) to move the car faster.

For a larger engine on the same car, you generally get worse gas mileage at lower speeds because the idle fuel usage is higher. But, as you go faster, the larger engine doesn't work much harder, so mileage doesn't drop as much. After a certain speed, a larger engine will be more efficient on the same car.

This is why turbochargers are now all the rage. You get the advantage of the lower idle consumption of the smaller engine, while still having the acceleration you need for emergencies and the better mileage at higher speeds.

Your mileage may vary...

[dtjohnson]

No one cares what the testing procedure is as long as everyone does the same test and it's repeatable. The purpose of the test is to provide a method for consumers to compare different models with respect to their fuel economy, not to provide a precise prediction of exactly what the buyer's fuel economy will be. Everyone drives differently. People warm their car up in the driveway, fill it up with heavy weight, carry lots of passengers, do a lot of long-distance driving, tow trailers, drive up and down hills, ride their brakes, accelerate briskly to beat their neighbor, drive at high altitudes, drive in cold weather, or whatever. Even more significantly, the energy content of 'gasoline' varies widely depending on how much ethanol it has (more is less) and what its boiling point range is. Just do the same test and do it in a way that someone else could repeat the test the same way and get the same result. That's all we need rather it's a 50 year old test or not.