Match your ratio to your habits
I have a 1959 Ford Galaxie 500, and I have been thinking about installing an AOD transmission behind its 390 cid engine. It initially came with a 332 cid engine with a Cruise-O-Matic. My question is this, is it necessary to change the rear end to try to match this set-up? If so, what type and gear ratio should I use? I used this vehicle as a daily driver for 20 years before I put it in the garage in 1995. That was a big mistake. It was not properly prepped for storage.
You should match your rear end ratio to the car’s transmission specifications and engine characteristics. And, of course, you should also take into consideration your driving habits and goals. For example, if you want to maximize off-the-line acceleration and don’t much care about fuel economy then you’d go with a numerically higher differential gear ratio. Conversely, if high-speed highway cruising and maximizing fuel economy are primary goals you’d choose a numerically low rear gear set.
Most people retrofit vintage cars with overdrive transmissions so that they can have a little of the best of both worlds— good acceleration, good highway performance, and better fuel economy than the car got with its original transmission.