Originally posted by moparmike
I thought it was an up-shift point.
you could use them as guides for upshifting, but the tachometer is much easier to use.
for downshifting, however, it is much more difficult to use the tachometer - you need to know the relative ratios between gears to determine if a downshift now will exceed redline in the next gear, and you need to calculate this quickly ($$$!!!) ... The markings on the speedometer face make this much easier to figure out.
as an additional point, note that there is no marking for 4th or 5th gears (if the car has a 5th gear). This means that the car is drag-limited in these gears, and there is no way that you can (normally) exceed redline in them.
i have a 5spd 190e2.6, and there are markings only up to 3rd gear. This means that at any speed in 5th (even top speed), I know that I can safely downshift to 4th - just have to rev-match to be easy on the drivetrain and synchros. I have not driven the car this hard though ...