Actually, I think that the speed limit to be lowered to 55 for any vehicle getting less than 30mpg at that speed. You could never enforce it, but SUV's (with the exception of Porsche & BMW) were not designed for high speeds.

55 is a better speed limit than 65, in terms of fuel consumption, but at 55, people drove 65. At 65, people drive 70. A law that everyone's ignoring is useless. Leave it at 65, but enforce it vigorously. It's not a recommendation, it's a law.