For optimal performance, you ideally want to shift between maximum power and maximum torque. i.e. once you've reached your power peak, you shift down so as not to drop the revs below the torque peak. That, of course, is best performance, and its really a bit much since the 318 developes its maximum kw at around 6000 which is really overkill, since the power curve is fairly flat. Nevertheless, on my 316, I find shifting at 3000 is fine, 4000 if i'm in a hurry, and if i need to 5k, but i havn't revved over 5500 - its really just pointless.