However, as always, I say make the changes that make the most sense.
When I heard a friend describe a relative as complaining about the 55mph minimum speed limit being too high because he wanted to drive his prius slower, I had to address mpg vs. speed here.
Each car has it's own mileage to speed continuum. However, for most cars it looks something like the graph below from fueleconomy where it starts to go down after a certain point.
A large factor in this is wind resistance, so cars that are like big air dams (pickup trucks cough cough) are going to have a lower speed at which mileage begins to decrease rapidly. More aerodynamic cars like the prius are going to have a higher point. A forum poster graphed his mileage in the prius here http://www.greenhybrid.com/discuss/f10/mpg-vs-speed-hybrid-driving-strategy-14276/ and claims that the speed of diminishing fuel savings is around 70mph.
So yes, you get better fuel mileage driving more slowly, but it really depends on the speed. So slowing down from 75 to 70 will save in just about any car. Slowing down from 65-60 in some cars, and 55 to 45 will not save nearly as much) Wind resistance goes up as the square of the speed so the higher the speed you'll use significantly more. This is why race cars are so intent on drafting (though I don't recommend drafting on semis, you don't get very good gas mileage when they have to slam on their brakes and you end up smashed, though I guess if you survive and are attached to their truck and your back wheels still work and you are going where they are...)
Plus as always here at "tha metric" we have to consider the other external costs. What about the drivers with road rage behind you that have to make an extra trip to the hospital for their blood pressure? c.f. Denis Leary's song lyrics: "I drive really slow in the ultra fast lane
While people behind me are going insane"
and would you do more for the environment by getting there 10 minutes earlier and spending those ten minutes planting a tree?
Answer due in homework bin by friday.
PS to clarify any agenda you may think I have or do not have, I generally drive the speed limit, as I think speed differential is dangerous but I also want to get there today and not use too much gas.
While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 60 mph.
You can assume that each 5 mph you drive over 60 mph is like paying an additional $0.30 per gallon for gas.