Obviously the idea of using a stirling engine to run a generator for an electric vehicle has been said, and i think that a stirling/electric hybrid would be the easiest and quickest application of a Stirling Car.
Some vehicles, like tractors, don't change speed often. Maybe something like that could use a transmission to change speed and just leave the engine running without over modifying it.
I wonder though, With an Aplpha Type Stirling Engine, with 2 pistons and a regenerator, would it be possible to slow the airflow through the regenerator? Would that slow the engine without taking too much power from it? Also if a Displacer type Stirling engine was used, could you use friction to slow the displacer piston, therefore slowing the mechanism? Slowing a displacer could be as simple as a handbrake. Maybe later when i get home i'll try to sketch an idea and post it... but would it work?