How did young James drop the convertible top on the Vette while driving it at 80+ MPH?? The pursuit starts out with the white convertible top up but soon the top is down, not something you can do in a mid 60s Vette.
Looking at the sequence again, it looks like it's probably cut together from shots from each end of a longer sequence in the movie - the landscape changes rather suddenly. So Kirk's probably been joyriding for a bit when the cop picks him up and the chase starts.