I’ve read many articles and studies claiming that modern cars are most fuel-efficient under 65mph, such as this one: http://www.fueleconomy.gov/feg/driveHabits.shtml.
However, I keep meticulous driving records, and I have discovered that the 2007 Toyota Corolla gets 8.3% better fuel efficiency by driving at what I will call “leftmost-lane flow-of-traffic speeds,” compared to “normal freeway speeds”. (Phrased carefully to not explicitly or publicly confess to experimental speeding… that shall remain ambiguous. Ha!)
Therefore I believe that the claims of these articles, studies, and websites are just a ploy to get people to drive more slowly and safely by claiming that they’re saving money on gas. It’s kind of like telling people “eating more vegetables will make you rich and famous”… not true, but perhaps a good way to trick people into eating healthy.
Of course, speeding is still a terrible idea, because it possibly entails: 1) endangering lives, including your own, 2) increased insurance premiums, 3) points on your driving record, and 4) transgressing the law.
Also, by my math, I could only afford to be pulled over and ticketed less frequently than once every 27200 miles in order to save money at all! (That’s the break even point where all the fuel money I save by speeding equals the cost of the ticket.)
In closing, I do not condone speeding. The purpose of this entry is not to promote speeding to save gas, but to simply point out the fact that these studies are probably intentionally making false claims about fuel efficiency as an excuse to just get people to slow down.

Actually I think it has less to do with an actual speed and more to do with a car’s optimal RPM…or so Ben tells me.