I've thought about this several times (that's how geeky I am, when I can't work, I think about things like this). Thinking about it the other day, I suddenly hit on a simple approximation made possible by the current prices.
The fuel tank of my truck holds 25 gallons. One cent per gallon would save me 25 cents. How far can I drive on 25 cents of gas? At $4/gallon, 25 cents buys one sixteenth of a gallon. If my vehicle got 16 miles per gallon, I could drive 1 mile on 25 cents. My truck actually gets about 18 miles per gallon and gas is not quite $4/gallon right now, but I'm just approximating.
So, if I drive 1 mile to save 1 penny per gallon I wasted my time (not to mention the wear on tires and such). My son's car only requires about half as much gas to fill up but he can go twice as far on a gallon of gas so the result is the same. 1 mile, 1 penny.
The bottom line is you can't drive very far out of your way to save on gas...