Here is my understanding (correct me if I'm wrong please)
P ........ i'm done, sleep well, beauty (no mphs)
R ........ go back, I forgot my valet, duh!!! (0-20mph only!)
N ........ this doesn't look good, a tow truck??? help!!! (no mphs)
D5 ...... step on it, lemme hear those 6x RPMs (good anywhere) (10-17 mphs city, 18-21 mphs highway - well don't expect high numbers if you keep listening to the revving engine!!!

)
D4 ...... step on it, but remember how expensive is gas (usually skip

)
D3 ...... step on it, i'm on the highway - LOOK OUT PEOPLE!!! (loose those 4 & 5 gears, who cares about those 22 mphs, we get 18 and are pretty happy with it, right?

)
2 ........ step on it (GENTLY), it's wet/snowy outside (or a very slow stop-n-go traffic), cause she stops faster (0-40mph)
1 ........... do not step on it at all, lemme get u out of that hole or ice skating ring (better press that VTM Lock and keep your speed under 18mph!!!)
my habbit is to switch to 2 from D5, when traffic slows down and then back to D5 when traffic frees up (during rainfall mostly); that may be another reason why i do not get highter than 18 mph, but the truck is more responsive that way and easier to slow it down.