You must consider the rear gear you are currently running?
Ex: 3.21 with the 200R4's .67 od gives you a 2.15 in 4th. Might be a little lazy even at highway speeds.
With the above ^^^^^^^^^and a 26" tire, you'd be about 1700 @ 60mph.
This is a great discussion on auto od's, but we need some info from the OP. Talk to us LJS30. What's your current gear and tire diameter? What's your budget? This could get pretty pricey and if your only gonna gain a few mpg you might be better off just using a taller tire and let it eat?