I have drag raced my creampuff a lot, before and after the supercharger I have always found faster times by shifting right at 6000 rpm on the factory tach. A lot of people have told me that I should shift at 5500 rpm, but whenever I shift at less than 6000 rpm I lose time and trap speed. I don't know how accurate the factory tach is, but mine seems to like 6000 rpm.