One thought is that the price index alone does not tell the full story. The average commute distance by auto increased by 22 % between 1983 and 1990 or about 3 % per year according to http://npts.ornl.gov/npts/1990/fat/tab6_11.pdf
So if that rate remains constant, the average commute distance by car should have increased by some 66% as compared to the 1980s. So to compare the actual cost of gasoline consumption, we should look not just at gas prices but also at gas consumption, which increased due to longer distances (I assume that mpg remains relatively constant due to SUVs and traffic congestion).
So by that logic, Jim's index figure of ca. 87 for 2005 should be increased by some 66% to 144 - way above the 108 peak figure for 1981.
Methinks that cars in the US are not a mode of transportation, but a peace of mind, an illusion of security and being in control, and status symbol - so people will be driving their gas guzzling SUVs even if they were to pay $5+ for the gallon.
Wojtek