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

Re:Simple explanation

[Anonymous Coward]

American highway speed limits vary depending on the state and location. It's usually 65 but inside a large city it is often 55. In the Texas desert it can be 85.

Re:Choice of average

[swalve]

When you are calculating the average of ratios (miles per gallon) and the denominator is the thing that changes, you have to use the geometric mean. If they used liters per 100km, then they would use the regular average.

Re:Simple explanation

[houghi]

http://en.wikipedia.org/wiki/Speed_limits_in_the_United_States
The highest speed limits are generally 75 mph (121 km/h) in western states and 70 mph (113 km/h) in eastern states. A few states, mainly in the Northeast Megalopolis, have 65 mph (105 km/h) limits, and Hawaii only has 60 mph (97 km/h) maximum limits. A small portion of the Texas and Utah road networks have higher limits.

So if you live in a 60MPH state, it is accurate. If you live in a 75MPH then it isn't.

So they should have different information for different states. And obviously for city driving as well.

Re:Units wrong

[xaxa]

For example, the maximum acceleration on either test is 3.3 mph per second.

It's hard to take a paper seriously when it gets the units of measure wrong.

What's the problem? That *is* an acceleration.

(The SI measure is ms^-2, metres per second squared, or metres per second per second. 3.3 (miles/hour)/second = 0.44704 m s^-2.)

Re:The Testing Process is Flawed

[Ironhandx]

Consumer reports tends to be a bit sensational. They do get put on a treadmill but the EPA numbers are also based on a circuit of real road driving. 3 tests are done and the average of those tests are then used as the EPA rating.

Additionally theres no such thing as a frictionless treadmill, and the treadmill they use is actually able to adjust its load to simulate real world resistances.

There are plenty of real reasons to bash the EPA ratings, there was no need for consumer reports to exaggerate and make shit up.

Re: Efficiency

[icebike]

Entire article is false, EPA changed the testing in 2008. Since 2008 they test mpg up to 80mph and accelerate at 8.5 miles per second, roughly 0-60 in 7 second

So I'm guessing you never bothered to read the article. (I know, what was I thinking).

The new standard is FULLY addressed in the article. Go read it.