The Oil Drum

31 comments on The Productivity of Vehicles

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apsmith on October 20, 2005 - 9:23am

Stuart - very interesting analysis.

There's some question in my mind about the averaging though - given the large differences between the different groups of vehicles, are you getting the right numbers for the improvement from the "weighted average"? And is the averaging within a sector even giving the right sort of numbers?

Even if the weighting is by highway miles driven, it still might not give you the right improvement ratio.

For an example, let's say cars get 20 mpg, light trucks 15 mpg, and other trucks 5 mpg. Let's say of every highway mile driven, 40% are cars, 40% light trucks, and 20% heavy trucks. Then, with those percentages you get a "weighted average" mpg of 15 (0.4*20 + 0.4*15 + 0.2*5). However, the actual number of gallons of oil used for one "average" mile is 0.08666 (0.2/5 + 0.4/15 + 0.4/20) giving a true average mpg of 11.5, not 15.

I.e. the real improvement may be much less than you would get from a simple weighted average of sector mpg's.

Also, the numbers for "transportation" energy use in your final graph presumably include air transportation (which has gone up considerably) and railway transportation (which has gone down), which would further shift the numbers to use of more energy per mile traveled. Given that transportation is using a greater fraction of the economy's energy that probably agrees.

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odograph on October 20, 2005 - 10:51am

I don't think anybody knows our effective fleet mileage, if that's what you are getting at.

The commonly referred to numbers are either - averages across one vendor's vehicles - or averages across models sold in a year.

To really find out who is driving what and how far ... we'd need surveys, which AFAIK we don't have.

I think I've seen more little old cars on the road since gas prices went up. I'm sure that is the case nationally. I'm also sure this shift in usage pattern won't show up in the fleet mpg statistics.

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solarfan on October 20, 2005 - 1:05pm

This is my first post, I've really enjoyed reading the excellent (and civil) discussions going on here.

apsmith is right about the weighted average. Just like if you ride a bike uphill for 1 mile at 10 miles per hour, then down the hill for 1 mile at 20 miles per hour, the average is NOT 15 miles per hour, because you spend much more time at 10 mph. The correct weighting is (0.667*10 + 0.333*20)=13.3 mph.

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Stuart Staniford on October 20, 2005 - 1:20pm

That graph is actually taken directly from the EIA. Looks like it's total miles over total gallons (which seems like the thing we want).

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odograph on October 20, 2005 - 4:04pm

I found something on how VMT is calculated. In some places, such as here in California, it is based on the numbers captured at vehicle smog inspections. States without such inspections are forced to use rougher methods:

http://www.epa.gov/ttn/chief/eiip/techreport/volume04/iv01.pdf

Of course in California:

"Beginning January 1, 2005, vehicles 6 or less model-years old will be exempt from the biennial Smog Check inspection requirement. For vehicles with registration renewals due in the 2005 calendar year, this exemption includes model-years 2000, 2001, 2002, 2003, 2004 and 2005."

Those guys are going to miss their data.