Since gas prices started going up, I've started paying a lot more attention to this kind of stuff.
Most of my commute to work, for example, is on a 4-lane divided highway that's posted at 65mph. It's a good road without much traffic, and typical speeds for me in the past have been ~75 and I won't pretend to not have done faster than that.
If I hop on the road and set my cruise at 65, I can easily hit 28mpg in my MKZ where more normal driving(for me) leaves me at 25-26mp . That doesn't sound much, but considering how many commuting miles I do a year, that adds up at $5/gallon. Leaving my house 5 minutes earlier easily makes up for the time I lose traveling 30 miles at 65mph vs. 75mph-in fact it sometimes gets me there earlier because before I'd often catch the morning Amtrak train headed to Chicago.
I've always been someone to do things like take my foot off the gas as soon as I see a light change on up ahead of me and wait as long as is safely possible to brake. Similarly, on the highway, I try to keep my following distances fairly long not only because it's safer but also because I can often easily and safely make a ~5mph drop if traffic or whatever dictates it without using the brakes.
I say all of that just to say that it almost seems to me like high speeds, tailgating, and things like that are even worse now than they were 6 months ago when gas was "only" $3.50 a gallon or so.