Even Simpler Than That...
Just so no one gets confused, I have been told that the roman numerals indicate shift sequence points, not gear numbers. In other words, "I" indicates the first shift point to second, "II" the second shift point to third, and so on.
There is nothing wrong with manually shifting an automatic if it is done properly. It's better done with a tach, and is done properly if you stay below the redline. The idea is to keep the engine in it's opimum power curve for better handling and performance. I manually shift my 300SD to keep the revs above 2500 RPM in order to have the turbo working. The trick is to note where the transmission shifts gears automatically in terms of speed and RPM, and then know just how far you can extend that range. The ideal is to match the speed of the crankshaft with the driveline by adjusting road speed and engine revolutions.
If you're unclear on the procedure, or just want to get the straight scoop from a real pro, go to your local library and see if they have any books by Bob Bondurant or Jackie Stewart. In fact, there's an excellent video by Jackie Stewart also.
"We drive into the future using only our rearview mirror."
- Marshall McLuhan -
Northern California Wine Country...
"Turbos whistle, grapes wine..."