In this week’s reading about resource (fossil fuel) depletion, Smil highlights four trends that are expected to contribute to strong growth in global oil demand in the near term; one stood out for me in particular:
“Finally, I must mention profound infrastructural changes in the US, the world’s largest oil market, where ex-urban (no more merely suburban) commuters are driving greater numbers of improbably sized, gasoline –guzzling vehicles and setting a most undesirable example to affluent populations around the world." 
I applaud the fact that Smil does not mince his words when expressing his opinion of this U.S. trend. On the following page, he makes a recommendation to help break the U.S. oil habit:
“Average fuel consumption of US private vehicles could be cut by one-third in just a matter of years by limiting the sales of gasoline-guzzling SUVs (classified as light trucks and hence exempt from minimum CAFE requirements for passenger cars) for urban use by raising the new CAFE rate (unchanged since 1987) from 27.5 mpg to 35 mpg; that is, to the level of fuel-efficient Japanese sedans.” 
Since this book was written, Smil’s words seem to have been taken to heart with the passing of the Energy Independence and Security Act (EISA) of 2007, which calls for a CAFE rate of 35 mpg by 2020. This is encouraging, but it’s safe to assume that the rule is not set at the pace that Smil envisaged. The question is, how did Smil come up with the statement that fuel consumption could be cut by one-third?
I mined the following data from the Bureau of Transportation Statistics for the year 2005 [Table 4-9, Table 1-1]:
Average miles traveled per gallon – 16.7 mpg (this is down from a peak of 17.1 mpg in 2001)
Vehicles registered - 241,194,000 (Current U.S. population is 301,621,157, which equates to 0.8 vehicles/person. There were 199,000,000 licensed drivers in 2004, which equates to 1.2 vehicles/driver!)
Fuel consumed - 179,100,000,000 gallons
Number of passenger cars – 136,568,083
Number of other 2-axle 4-tire vehicles – 95,336,839 (assumed to be light-duty trucks, i.e. SUVs, minivans, pickup trucks)
I pulled the relevant CAFE standard numbers from an article on MIT Technology Review :
Current standard for cars – 27.5 mpg (set in 1984)
Current standard for light trucks (minivans, SUVs, pickups) – 22.2 mpg
Future standard encompassing entire fleet – 35 mpg
Using the fleet numbers, we find that light trucks make up ~40% of the motor vehicle fleet, while cars make up 60%. If we assume that all vehicles currently meet CAFE standards, the fleet average would be 25.4 mpg; this is 8.7 mpg greater than the actual average (16.7 mpg). If we assume a similar difference between the standard and actual average when the 35 mpg standard goes into effect, the fleet average would be 26.3 mpg. Compared with today’s average, this results in ~38% reduction in fuel consumption (also assumes no increase in car fleet size or distance traveled per vehicle).
Smil suggests that this 1/3 cut could somehow be achieved in a few years. Even if the 35 mpg CAFE were implemented today (basically banning SUV and light truck sales), it could take years—perhaps decades—for the existing fleet to see any appreciable improvement in average fuel consumption. Regardless, I think most of us would agree that one of the quickest and easiest ways to cut oil use in the U.S. is through improved fuel economy and reduced commute time. I'm open to other interpretations of Smil's SUV recommendation.
 Smil, V., Energy at the Crossroads. 2005, Cambridge: The MIT Press. p. 206-207.