Some help with rear end gearing

Question:

I have a 1959 Ford Galaxie 500 and have been thinking about installing an automatic overdrive transmission behind a 390 cid engine. It initially came with a 332 cid with a Cruse-O-Matic. Is it necessary to change the rear end to try to match this new setup? If so, what type and gear ratio should I use? I used this vehicle as a daily driver for 20 years before I drove it in the garage in 1995. That was a big mistake because it was not properly prepared for storage.

Answer:

Whether it’s advisable to change the rear end gearing at the same time you install an automatic overdrive transmission depends on how you intend to use the car and what gear ratio is in the rear end now.

For ordinary driving on both highways and surface streets (as opposed to drag racing) most people choose to go with a relatively tall (which means a numerically low) gear ratio. Something in the neighborhood of 3.30:1 behind a 390 cid engine and automatic will give you good off-the-line acceleration, excellent high-speed highway performance, and relatively good fuel economy.