(yes, i realize this is a thread from 2012)
I've been tracking mileage for just over a year on my car, and this week, I decided to do a 1-tank experiment and slow down by 10mph. I typically drive 75 pretty much everywhere. The idea has been to drop down to 60-65 and see if it really has that much of an effect on fuel economy.
I'll post conclusive results after my next fill-up, but preliminary results appear significant. I normally get 330-380 miles on a tank of gas, but it's not uncommon for the first half tank to be 200 miles. I expect the gauge is inaccurate, but I also believe it to be consistent. Anyway, at the half-tank mark doing 60-65, I had over 280 miles on this tank. I'll do the real math after I fill-up again, and then maybe run a second full tank through to confirm the numbers.
But what I'm saying is, the best way to save gas is just to slow down... Drag increases exponentially with speed. 10mph slower is as much as 23% more efficient (per the internet). We'll see if that bears out in real-world driving conditions.