The idea is that the most cost-effective speed to drive your car needs to be based on your hourly earnings and your car's mpg. Factor in the cost of fuel and the speed at which your car gets the best mpg, and you have Rhett's optimal commuting speed calculator . Some of the problems with the calculator are due to the need to use old data. Also, super-efficient vehicles kind of blow the model out of the water. Rhett says there are improvements coming to the calculator, so feel free to chime in with those helpful hints.
[Source: Trivial Physics]