Its really not that difficult to pick an optimal gear ratio if you have a BSFC chart for your engine.
First off, you have to figure out how much horsepower your car demands at 65 mph. Sorry, I don't have the equation handy, but its findable (you'll need Cd, frontal area, and speed). I think I got it off autospeed at one time. For example, my Matrix takes roughly 14 horsepower to stay at 65 mph.
Second, you'll want to plot a bunch of points on the BSFC chart to see where that lands you. To do this you need to convert your horsepower to torque at a specific rpm.
torque = horsepower * 5252 / rpm
So lets plot a few points on the Saturn BSFC chart since I don't have a chart for the Matrix. This is obviously going to skew things, but it should get the idea across.
RPM....Torque @ 65 mph
1000...73
1500...49
2000...37
2500...29
3000...25
3500...21
4000...18
So, you could actually gear it all the way down to cruise at 1000 rpm and that would give you the best fuel economy. However, you would be still only be at 50% load and you would have next to no power avaliable should you need to pass or keep speed going up a hill. That may not bother some of you (it wouldn't bother me much), but it would bug the majority of buyers out there which is one reason why auto companies have not done this except on their high mileage cars (Metro xfi, Insight, Civic VX/HX, etc.). Even then they have been conservative so as to provide you with something that is easy to drive vs incredibly efficient. This is also the reason why you see such huge gains in FE from P&G.