My old 89 had a T5z gear set in it with a .63 5th. I had 4.10's in the rear with 26" tall tires and it cruised very nice at 75mph. I think if you put a T-56 in the car with a .50 6th you will almost never use it with 3.73's. With 26" tall tires at 75mph that works out to 1800 rpms, that is...