The FSM says to use a tach that has graduations of 50 rpm or smaller, which is ridiculously fine compared to the dash tach. I have not been able to find any kind aftermarket tach out there with this capability. A digital tach just shows you a snapshot evey time it updates, and it's different every time if the idle isn't dead smooth. I think the old-style analog maintenance tachs would probably do it, but they're long gone, mostly. If any of you guys are actually doing this in your garage, what do you use?