Hello Gp, You didn't say what size tires you have, but you are right. If you are not doing any open road running, it doest matter. If you did add the OD, you would only be turning about 2100 at 55. But , if you're not planning on touring w/ the car, the expense isn't really justified. I like to go places w/ my cars occasionally, IE, I would kind of like to drive it to Carlisle, but 3.91s are a little steep for prolonged 75 mph cruising, not mention, the fuel bill will be steep too. I'm still thinking about it though. My '65 runs about 2800 at 60 w/ 3.21 & 225-60-14 rears. The Chally would be somewhat higher I'm sure. Ovmf