Car numbers have been based on constructors championship position since 1996 but in continuing what I can only assume to be a very poorly timed April Fools joke, the FIA is allowing the drivers to choose their own car number in 2014.
The number '1' will continue to be reserved for the reigning champion but numbers 2 through 99 will be available for the choosing. In a case where a conflict occurs, preference goes by championship finishing position.
In other bizarre news, the FIA is considering renaming the series as the Rolex Formula 1 World Championship, powered by Infiniti®.*