I was originally planning on putting a 5 speed in my 52 Plymouth but now I'm looking at a automatic for a nicer cruise. I am building this car into a sled plan on making it a good cruiser with some power (around 400 horse). I have a 904 lockup transmission and when I searched it there are all kinds of threads about how to make it a non lock up. Why is that? I would assume the lockup trans woud help with fuel mileage even if marginal. If the lock up stall RPMs aren't at cruising speed I don't know why it is a problem. Any elaboration on which is better and why, the lock up trans has a better gear ratio so I'd like to use it.