From westerly direction to Stuttgart via AB A8/A81 and B14:

S-Bahn-Stop Österfeld
Trains S1, S2, S3 use underground parking "Unterer Grund", 514 parking spaces,
Closed between 01:00 and 04:30 o'clock no exit possible!
Use 2-Zone ticket travel to center (Stadtmitte) in 9 minutes continue on to stop 'Bad Cannstatt'

PDF - local map Österfeld

Link - more information at VVS

From south to Stuttgart via B27:

Stadtbahn-Station Degerloch
Parking lot Degerloch, 726 parking spaces, 24 h exit possible, Security Tel. (07022) 41091
Directly adjecent to stop 'Degerloch Albstraße' - next to Bundesstraße B27 - the SSB has constructed a Park & Ride facility. Special offer: The parking fee ticket is also a day ticket train rides into the city center. 

Link - more Information at SSB