Accuracy of Tachometer with Partially Engaged Clutch?

Could you please tell where the tachometer reading is physically measured (for example, on the crankshaft)and whether it provides an accurate measurement when the clutch is only partially engaged? More specifically, on a couple of occasions I have gotten "lost" in the 6-speed gear pattern and have let out the clutch slowly to monitor the rpm and have the opportunity to disengage the clutch if the engine begins to rev higher than I would prefer. On one occasion it turned out that the car was travelling fast enough to rev the engine to 5000 rpm in the gear I selected but as I was letting out the clutch slowly, the engine only revved to 4000 rpm (as indicated by the tachometer) before I was able to disengage the clutch. Did I indeed successfully prevent the engine from revving above 4000 rpm or does the tachometer not measure engine speed correctly under these conditions? Thanks, John

The computer (DME) of the 996 can monitor and prevent overrevs if you are stepping on the accelerator very well. The DME sees the rev count and causes a misfire so the car will not continue (past redline). You can actually see the number of overrevs with a PST2 -- these are listed as stage 1 errors and do no harm. I've even heard some Porsche mechanics say you can never blow the engine that way... You can however overrev the engine (and do damage) by dropping the clutch in the wrong gear. These are read as stage 2 errors and could be seen by Porsche or a dealer as abuse (if there is engine damage). There is nothing the DME (or anything else) can do to prevent overrevs by downshifting. You are better off to put the clutch in and find neutral and then find the right gear... or even pull over and stop and find the right gear. Just IMHO.