Driving slower in fact does produce better gas mileage. When a crew chief in NASCAR wants his driver to save some gas, how do you think that driver goes about it when he is leading a race? And have you seen how some speed and weave in and out of traffic? They would indeed be idiots when they are putting others in danger.
Again, it depends on the specific vehicle and speed. Driving slower does not necessarily produce better gas mileage. If we started off in the same type of car with the same amount of gas and you drove 5 MPH while I drove 75 MPH, who do you think would cover more distance before running out of fuel?