Thursday, December 20, 2007

Saving Gas

Global climate change isn't particularly my topic here but nevertheless there's a post on Gristmill today that starts with the words, "This never fails to fascinate me," and indeed it fascinated and amazed me.

If we want to reduce gasoline use, we can make a huge gain by concentrating first on vehicles that get very low mileage. Obvious. The amazing thing is that low mileage vehicles don't have to be traded in for high mileage vehicles to get the huge gain; trading in an SUV that gets 16 mpg for one that gets 23 mpg saves twice as much gas as trading in a car that gets 32 for a hybrid that gets 47 (of course, trading in a 16 for a 42 would be even better). Read it
here and look at the graph. (Update: Read through the comments on Gristmill for more good stuff.)

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Blogger Bloggo said...

Good find. This is fairly obvious to people when you point out the logic but it does not dawn on most of us until it is brought to our attention.

6:06 PM  
Anonymous Bryan said...

There's a good lesson in that Grist post. Thanks for finding it. I believe that people tend to think linearly: Double the input and you get double the output. The problem is, not everything in the world responds in a linear fashion.

Case in point: If you reverse the curve in the Grist post, so the right side shoots up into space, you'd have a good approximation of what happens to gas mileage (mpg) as your speed increases.

Taking a 100-mile trip and driving 60mph does not use same amount of gas as a trip taken at 30mph. It takes a lot more gas to drive it at 60mph, even though you arrive in half the time. It takes increasingly greater amounts of gas to drive that 100 miles at 70, 75, 80mph, etc.

So there's gas to be saved driving on the LIE and parkways a few mph slower, plus eliminating the jack-rabbit starts and not "driving" all the way up to the red light.

Every car should have an mpg indicator and a tach (better to see the shift points on your automatic transmission in order to keep the rpms as low as possible).

7:29 PM  
Blogger Tom Andersen said...

Is lowering the speed limit even part of the debate these days?

6:54 AM  
Blogger Sam said...

No, nobody I know of is talking about the activity side. It doesn't make much sense, either. The optimum vehicle speed in terms of emissions and fuel consumption is somewhere is a "sweet spot" about 47 MPH. At very slow speeds, lugging can occur (start and stop "jackrabbit" driving). Once you get close to 60 the MPG becomes much worse simply due to wind resistance, a function of the power of two (squared).

Then the issue becomes one of acceleration. In the past the EPA developed driving cycles that had an acceleration we called "the grandmaw cycle" because it was so gentle. Naturally, with all these high testosterone muscle cars and SUV these days, they're wasting fuel on rapid acceleration and getting nowhere just as fast.

But the most alarming studies of driving habit show that people are either lost, driving for fun, or simply going in circles. Other than the morning and evening business commuter rushes, GPS equipped vehicles showed that the operators were indeed driving in circles with no apparent destination. This was controversial because it showed that the drivers were not only teenagers and lost old people, but taxi drivers and commercial truck drivers as well.

I guess it's an American tradition!

2:11 PM  

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