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?
Well I am running 225/70/14's and I believe I have 3.55's for gearing. My goal is to make my Dart a highly functional yet very fun to drive street machine. That means I plan to actually use it for forty mile round trip commutes about three times a week. Keep in mind this will be in Southern California freeways so I need to cruise comfortably at 70/75MPH. I have a mild 318 with a 260H with an edelbrock carb and intake combo.
As for funds I am by no means a blue blooded high roller. I'm thinking in the range of two grand or so for this project. However if more money is needed to do this right I'll just have to save longer.
