# mpgillusion.blogspot.com

Just a short post to draw your attention to mpgillusion.blogspot.com.

Update: now on RC

1. #1 silence
2008/09/18

That’s a concept which has been floating around in the US for several months now. See for example http://www.npr.org/templates/story/story.php?storyId=91703607

2. #2 andrewt
2008/09/18

Australis uses L/100 km, I thought much of the (metric) world did this?

3. #3 Eli Rabett
2008/09/19

The calculation assumes miles driven is inelastic. It is crap.

4. #4 koen
2008/09/19

What’s still missing in the equation is the amount of ‘gas’ used to build the car (and to transport the components and/or the car halfway around the world etc).

That is, the move from a less fuel-efficient car to a more efficient car requires to drive thousands of miles only to “offset” the amount of greenhouses gases you put in the air by acquiring the car. And if you want to reduce your driving to lower your carbon footprint, it will take longer in time also to offset this.

Or stated otherwise, it is almost never a good idea to buy a new car. Keep and maintain the one you own will save more on greenhouse gas and your wallet than buying whatever alternative.

And yes, it will create a move of jobs away from car manufacturing to car repair and maintenance.

5. #5 MattXIV
2008/09/23

The calculation assumes miles driven is inelastic. It is crap.

The numbers still favor improving the low MPG car even when you take elasticity into account. For the example at RC, % change of Q high MPG car = .67 % change of Q low MPG car if the elasticities are the same. Plug in the short term elasticity of demand for gas as an estimate of the elasticity of demand for miles travelled in terms of gas (-.26 from here) with the car as a given (it will actually be lower since some people are car shopping in a given short run, but we’ll be conservative). You get the low MPG car driving around 11,000 miles and using 616 gal vs 830 gal before for a savings of 214 gal. High MPG car drives 11,500 miles using 253 gal of gas compared with 400 gal before for a savings of 147 gal. If my math is correct, elasticity of demand would have to be -1.67 for the two to even out.

6. #6 TrueSceptic
2008/09/25

I think this point is a little misguided. Yes, it is normal in continental Europe to quote car fuel consumption in l/100 km, but the result is the same: we want to know how far we can get on a tankful and how much it costs us.

The following changes (rounded) will all result in a saving of 1 gallon per 100 miles but note the %age improvement:-
67 -> 200 +200%
59 -> 141 +141%
50 -> 100 +100%
41 -> 71 +71%
33 -> 50 +50%
26 -> 35 +35%
20 -> 25 +25%
15.0 -> 17.7 +18%
11.1 -> 12.5 +13%
8.1 -> 8.8 +9%
I would argue that the saving of 1 gallon does *not* have the same meaning to someone towards the bottom of that list as it does to someone near the top. We are more likely to appreciate a saving as a %age. Reducing my fuel costs by 10% would be no big deal, but halving them would! By the same token, shouldn’t a profligate user be expected to make the same effort as the already abstemious? The %age difference represents that effort; the 1 gallon reduction does not.