It depends on what kind of driving you are doing. I notice for slower pace driving that around 4-5k is where it is happiest, but for a faster pace I run it right up to 7.5k (redline is 8k, sometimes I bang off the rev limiter) and shift quickly (especially from 1st to 2nd) or you might get some roughness. As far as speed, I don't recall exactly but 1st is good up to like 10mph (cruising speed, shifting speed is obviously higher), 2nd is good to like 20 or so, 3rd up to like 30, 4th up to 40, etc. I know that when I am just cruising around 40 I shift into 5th because it is right at 3k in 4th. The speeds that I stated might be a little lower, like 2nd to 18, 3rd to 26, 4th to 35, but I just like to keep the RPMs between 2k and 3k when cruising so I don't know exactly. The first speeds are good for downshifting, so if slowing from 50 to 35, I would go into 4th.