I HAVE 1963-98 with original rear end & a 1965 model 425 ,the gear ratio is not right for the highway now with 14s that came on it.any segestions other than putting 15s on it .do i need to change rear gears?

 

Is your question that you want to lower your engine RPMs on the highway? First of all, you should probably determine what ratio you actually have. Second, WHEEL diameter is irrelevant unless you are running the car without any tires. You can get 15" tires that have a smaller outside diameter than some 14" tires. Why does everyone fixate on the WHEEL diameter? Worry about the TIRE diameter.

 

Not sure why you would say this car is not right for the highway since it has it's original rear end. These cars were built for highway driving. It probably has something like a 3.08 gearing which is a nice compromise for a heavy car like that.

These cars were not built in the era of fuel economy being that important. If you're looking for better mileage? You probably could change to a different gear or look at a different transmission. Like JP said, check what you have already. Maybe going with oversize tires will be cheaper for you? If you do that be sure to compensate your speedo with the right speedo gear or carry GPS in the car so you know your proper speed.
I just went the other way. I had 2:73's in my car and changed to a series 3 posi with 3.42's and have the proper speedo adapter gear but I'm not concerned about mileage either.