The most fuel-efficient speed to drive is as slow as you can in the highest gear -- probably around 40-50 mph for most cars. Above 60 mph mileage really suffers from wind resistance as you go faster. I'm not sure, however, that people are actually driving slower now that fuel is so expensive. People just aren't that smart.

Every vehicle has it's own "Sweet Spot" for the best MPG. The best way is to experiment with your car and see. I have a display that will show average MPG on my G6. I started at 55 on the highway and traveled about 20 miles on the expressway, doing a reset as I entered the e-way. I then noted the MPG as I exited. I repeated this same trip at 60, 65, 70. 75 and 80 MPH. My vehicle got the best MPG at 70.

Im not an expert but I have read that when you're in your highest gear with the lowest rpms possible, that is when you're most fuel-efficient. (high gear + low rpms = less gas used) On the highway at least.

Well, that's not what the people are like here in Houston....they use the highways as racetracks regardless of gas price, LOL.