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January 22, 2008

How Many Priuses Does It Take To Cancel Out A Range Rover? Energy

We Prius owners tend to view hybrid SUVs like the Chevy Tahoe Hybrid, which gets only about 20 MPG, as some kind of sick joke. But the laugh's on us. If you really want to increase fuel efficiency, the place to concentrate your efforts is at the bottom end. Nerdblog explains.

Consider a hypothetical Range Rover (10 MPG) vs. a Prius (60 MPG). Naively, we think the Prius cancels out the Range Rover, since the average of 10 MPG and 60 MPG is a respectable 35 MPG. But this is just bad math. The thing we want to minimize is gallons used, so we should be averaging not miles per gallon, but gallons per mile.

Suppose, for example, the Range Rover and Prius each drive 60 miles. The Range Rover would use 6 gallons, the Prius one. So we've used 7 gallons to go 120 miles. 120 miles divided by 7 gallons is about 17 MPG. The Prius hasn't come close to cancelling out the Range Rover.

How many Priuses would it take to cancel out the Range Rover (i.e., move the average up to 35 MPG)?

Answer: six. Do the math.

Converting your 60 MPG Prius into a 100 MPG plug-in hybrid makes almost no difference in the fleet average, because your 60 MPG Prius is already using such a relatively small amount of fuel. It would probably do a lot more good to take the time, effort, and money that would have been required to do the conversion and use it to campaign for higher CAFE standards. Increasing the efficiency of the least efficient vehicles is what really matters.

[Thanks, Dennis]

Posted by Jonathan at January 22, 2008 09:43 PM  del.icio.us digg NewsVine Reddit YahooMyWeb

Comments

I'm a little surprised with this post. It is all so connected: we need CAFE and Hybrids. The journey is long and this strikes me as something of a quarrel about numbers. Statisticians were absent on the Left when this debate began and we needed them. We need them now, as well. But this Prius quarrel strikes me as marginally idiotic.

Posted by: regular at January 24, 2008 01:31 PM

Sorry if I made it seem like a quarrel. Not how I was thinking of it. It was more of an Aha! moment about how the math actually works.

Of course we need hybrids -- I own a Prius myself. But it's important not to fool ourselves. All the Priuses in the world are going to delay the onset of peak oil and global climate change by what? A few seconds, maybe? It's not enough to buy a Prius and feel like our work is done. It's not. Not remotely. As you said, it's all connected, and it's all needed. It's just helpful to know where we'll get the most bang for the buck.

Posted by: Jonathan at January 24, 2008 02:55 PM

I take exception to Nerdblog's logic.

If two Range Rovers drove 60 miles, then 12 gallones would be used. So by switching one of the Range Rovers to a Prius, we're using only 7 gallons, and we've saved about 42% of the fuel. A 42% savings sounds great to me.

Nerdblog rightly states that "The thing we want to minimize is gallons used, so we should be averaging not miles per gallon, but gallons per mile."

But then nerdblog goes on to talk about MPG, instead of GPM.

To sway public opinion, the public needs to see more hybrids, PHEV's, and EV's on the road, in the parking lot at the mall, or in their neighbor's driveway.

If I start driving an Aptera (aptera.com) or a Tango (commutercars.com) around town, I'm definitely going to raise awareness. (Though I'd rather do this in a somewhat more conventional-looking car.)

The more people that convert their hybrid to a PHEV, the sooner the car companies will start building PHEV's.

Posted by: PaulK at January 25, 2008 01:04 AM

Umm.... It takes one if the range rover driver trades in for the hybrid. Maybe a few nice bumper stickers. "SUV owners really hate their kids".

Just a thought

Posted by: MarkG at January 28, 2008 10:19 AM

Paul, a couple of things. Your analysis assumes each Prius replaces a Range Rover -- kinda the absolute best case. And the point isn't that MPG is a useless measure, it's just that you can't get the right answer simply by averaging MPG figures.

As for the importance of hybrids and PHEVs for conssciousness-raising, I couldn't agree more. One of the big reasons I bought a Prius rather than a Honda Civic hybrid, say, was that I wanted my car to be instantly recognizable as a hybrid. I wanted to help raise awareness in the people around me as I drive around.

Your point that plug-in conversions will hasten the day when car companies sell PHEVs is a good one. Point taken.

Posted by: Jonathan at January 29, 2008 10:10 AM