PostgreSQL 9.3: Minimize locking when updating a large table with primary key

Question

I am building a DB solution to handle a fast stream of online events.

Each event UPSERTs a row in a location_update table, using its car_id primary key. Since there are very few new car_ids, the vast majority of events trigger an UPDATE.

Since the table very frequently read by SELECT statements, I would like to minimize any locking and blocking delays on the table, and especially prevent a single UPDATE from blocking a large number of SELECTs.

How do I minimize Read/Write locking time on a table being UPDATEd using a primary key?