"Sen. John Warner, R-Va., asked Energy Secretary Samuel Bodman to look into what speed limit would provide optimum gasoline efficiency given current technology. He said he wants to know if the administration might support efforts in Congress to require a lower speed limit."
The more important issue, while Congress obsesses about gas prices, is the safety effect. The results on this are complex.
Lowering speed limits on freeways, with enforcement, will encourage fast drivers to drive on rural roads with less enforcement and with less safety features, and likely drive up crash and fatality rates systemwide (though perhaps lower it on freeways).
Important research on this is by Charles Lave and Patrick Elias (1994) Accident Analysis and Prevention 26(1) 49-62 who studied the effect of raising the 55 MPH speed limit to 65 MPH in 1987.