Weatherdem's Weblog

Bridging climate science, citizens, and policy

Car mileage evaluation criticism

Leave a comment

The Denver Post Editorial Board took a stance on car mileage based on a recent Consumer Reports (CR) article.  Let me state at the outset that I regularly use Consumer Reports rankings as part of my purchase decision-making process.  That said, no testing is ever 100% complete and is much less regularly communicated well to non-experts.  At issue: CR performed independent tests on cars and calculated different miles per gallon values than those the EPA provided.  Should the EPA update their testing?  Perhaps, but the Editorial Board and CR didn’t provide an overwhelming case to do so.  Let’s look at what each entity said.

First, the Post:

Consumers could very well feel deceived by the numbers, but there are other issues at work.

Hybrids with just a single occupant can zip past traffic using high-occupancy-vehicle lanes in some parts of the country — including Colorado — because of their superior efficiency. The idea is to support, through public policy, efficient vehicles that generate less harmful emissions. But if they’re really not substantially more efficient, it’s neither environmentally beneficial nor fair to drivers of traditional vehicles that may, in reality, get similar gas mileage.

There are key parts to this section that I want to highlight.  What does “substantially more efficient” mean?  What value efficiency is enough to warrant public policy?  In the Denver area, suburban drivers love their SUVs and trucks.  So to start, I’ll compare an SUV, a truck, a sports car, and a 2010 Prius.  And I’ll start with the location the Board identified as a policy recipient: the highway.  The EPA’s ratings for these four vehicles are: 15, 18, 26, and 48mpg (highway).  Should Prius drivers receive plicy support for driving a vehicle that averages 2-3X as many mpg as the majority of other vehicles?

Let’s change the argument a little to better match the spirit of the article and consider the “combined” fuel efficiency.  This designation accounts for street driving and highway driving.  The same four vehicles have the following combined ratings: 13, 14, 21, and 50mpg.  The complaint that CR and the Board has is the hybrid value is too high (based on their own testing protocols, which aren’t detailed very well).  So let’s change the hybrid’s EPA combined value with CR’s “overall” value: 44mpg.  Should we direct public policy toward cars that get 2-3X as many mpg as the majority of other vehicles?  Note that the comparison, and thus my argument, didn’t change.

The argument that CR made is that their hybrid vehicle test values differed from EPA values by more than 3mpg on average.  Yes, 6mpg is a difference.  Are consumers being tricked?  I don’t think so.  And this is really where I split with CR.  Here is their take:

Overall, fuel-efficiency shortfalls have narrowed considerably over the years. When Consumer Reports conducted a similar study in 2005 that compared our gas-mileage results with the EPA estimates, we found that most cars got significantly fewer mpg than their window stickers promised. Conventional gas-powered vehicles missed their EPA estimates by an average of 9 percent, and hybrids by 18 percent.

So it isn’t as if CR is bashing hybrids; far from it.  All of the EPA sticker values differed from CR’s.  That isn’t surprising since CR tested vehicles differently.  One vital question: which test best mimics real-world driving?  Most people drive very aggressively (hard acceleration and braking), which has a big impact on gas mileage.  Are the CR values too high?  What if we consider additional factors: heat and cold, wind and rain.  Those happen in the real world.  But not in the CR tests.  Isn’t it interesting that a consumer advocacy group is challenging the EPA’s tests for being unrealistic when they themselves didn’t test vehicles in real-world conditions?  How far off are CR’s values?  We don’t know.  Should we reengineer public policy in the face of CR’s unrealistic tests?  For what purpose?  Should the EPA and CR develop a more rigorous testing protocol – perhaps one that both entities can perform and therefore directly compare against one another?

I have another problem with the CR quote.  Note the bolded words.  EPA doesn’t promise drivers that they’ll attain the sticker mileage.  In fact, the EPA goes out of its way to emphasize and explain their values are merely estimates.  That’s not a promise, not even close.  It’s annoying that people read too much into things.  In fact, CR could do what I did: check the EPA website for common misconceptions.  The ninth most common:

9. Fuel Economy Label The EPA fuel economy estimates are a government guarantee on what fuel economy each vehicle will deliver.
The primary purpose of EPA fuel economy estimates is to provide consumers with a uniform, unbiased way of comparing the relative efficiency of vehicles. Even though the EPA’s test procedures are designed to reflect real-world driving conditions, no single test can accurately model all driving styles and environments. Differing fuel blends will also affect fuel economy. The use of gasoline with 10% ethanol can decrease fuel economy by about 3% due to its lower energy density.

Like I said, the EPA goes out of its way to explain what its published values are.  CR and the Board should have done 60 seconds of checking before they called out the EPA for deceiving consumers with “promises”.

A more appropriate target are auto manufacturers, who know what the tests are and try their best to optimize the results.  The same entity that has a financial interest in optimizing estimated mileage should not test vehicles’ mileage.  Like I wrote above, this is where CR and EPA should work together to independently test vehicles.

But as far as the basic argument goes, hybrids do get better overall mileage than other vehicles.  They get the best mileage if drivers drive them where manufacturers intended them to drive: in stop-and-go city traffic.  But they still drastically outperform their competition in highway driving, enough so that I think current public policies provide nearly the correct incentive for drivers to think of another dimension when choosing which vehicle they will purchase.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s