I put P235/75R15 Firestone tires on my 65 New yorker, with 15" alloy wheels. It handles and rides GREAT !! Also the tires don't look too big for this car. Your 66 and my 65 almost the same car. Personally, I wouldn't use less than a P225/75R15. These cars weigh about 4300 lbs, before you fill...