From Wikipedia on fuel economy in automobiles: "The power to overcome air resistance increases roughly with the cube of the speed, and thus the energy required per unit distance is roughly proportional to the square of speed." Furthermore, this is the dominant factor for large velocity.
So let's say your fuel usage, measured in fuel used per unit of distance (say, gallons per mile), at velocity v, is kv2. (k is some constant that depends on the car. A typical value of k, for a car using 0.05 gallons per mile at 60 mph, is 0.000014.) Let's say you value your time at a rate c -- measured in, say, dollars per hour, and the price of fuel is p.
Then for a journey of length d, you'll spend dpkv2 in fuel, and cd/v in time. Your total cost is
and differentiating and setting f'(v) = 0, the optimal speed is (c/2pk)1/3. The cost of the journey at this speed is
So according to this model, if you value your time more you should go faster; not surprisingly your value of time c and the price of fuel p show up only as c/p -- effectively, your value of time measured in terms of fuel.
Also, the optimal speed doesn't go down that slowly as p increases -- it only goes as p-1/3. But a doubling in gas prices still leads to a 20 percent reduction in optimal speed -- perhaps roughly in line with what people are suggesting. Taking c = 10, p = 4.05, k = 0.000014 gives an optimal speed of 45 miles per hour, although given the crudeness of this model (I've assumed that all the fuel is used to fight air resistance) I'd take that with a grain of salt, and I won't even touch the fact that different people place different values on their time and get different fuel economy. We can't just let everyone drive at their optimal speed.
Besides, part of the whole point of this is that if we use less fuel, demand for fuel will drop significantly below supply and oil prices will go down. So to forecast the effects of a lower speed limit I'd have to factor in that gasoline could get cheaper -- and let's face it, I can't predict the workings of the oil market.