Just swapped the 41t rear sproket for a 38t on my KZ650. What a difference. 70mph was about 5100 rpm, now about 4400 rpm. I live a M/H park with 10 mph speed limit enforeced by a lot of speed bumps. Used to shift up and down from first to second all the time in the park. Now I can stay in first. I can't really feel any difference in acceleration. Just seems like I added a 5th speed.