A couple of interesting tables.
Motor Vehicle Fuel Consumption and Travel in the U.S., 1960–2005 — Infoplease.com
The bottom stat was most interesting to me. Look at the spike in avg fuel useage in 1996, after the speed limits were raised.
Another chart showing fuel consumption by type.
BTS | Table 4-5: Fuel Consumption by Mode of Transportation in Physical Units
Hard to guess the impact of the 55 limit.
I saw several other graphs showing overall oil consumption. When compared to the price of gas, total consumption reflects the affect of relative price more than any other factor. The 2 major dips reflect the oil embargoes in the '70s. Otherwise, consumption increases until stopped by a temporary rise in prices, then continues.