rctice, I'm not a 1/12 guru by any means - I just came off of a 12 yr "retirement" . I ran my first stock race in over a decade tonight. I started off with a 30 pinion and 96 spur with 1.77 rear tires with a monster stock with F brushes on a mildly technical mid 12 sec lap track. I went up to a 33 pinion and gained 3 tenths! Had 30 secs run time left with really old 3300's. With punchier batteries - your gearing may be much different.
Moral of the story = gearing seems to be pretty sensitive with stock. Do what your lap times tell you. MANY variables at play, not the least of which how you roll into the trigger.