If you are interested send me a PM with your e-mail address and I can e-mail you info I have on rear axle codes, trans codes, paint codes, and stuff. There were actually many different axle ratios available in '66. The most common was probably the 3.00. <a href="http://www.ridgenet.net/~biesiade/Fordrears.html" target="_blank">Click here</a> for a link for 8-inch or 9-inch question. The site gives gear ratios by ID tag on rear. The paper I have gives it by vin breakdown. 3.00 is basically economy highway gears. If you want a little extra kick without overdueing it 3.50 would be pretty good without an overdrive trans. I have 3.70's in mine and she revs pretty high on the interstates (I also have stock diameter tires which makes a difference). I also switched to a 9-inch rear due to the lack of available parts for the 8-inch. Gears is something you'll have to decide on based on what you really want out of your car. Mine does mostly local and back road cruising so I don't mind the higher rpm's that much, but if I was doing a lot of highway driving or long trips I would probably prefer between 3.25 and 3.50 with my stock tires. But if you're looking to rip from light to light and limited highway use I think 3.70's make a nice choice.