2nd. gear in the 420/465 is approximately equivalent to 1st. gear in a 3 speed. It's also synchronized, which allows you to drop the trans into second on a rolling stop, which can't be done with 1st. on an early model 3 speed. Most people stay out of granny gear unless they're getting a heavy load moving or they're starting on a steep upgrade. With the 4.56, third gear should be good for any speed from 10 MPH to 30 or so.
Jerry
