Templates not working for custom content type

I’m trying to create a custom content type for a client and I cannot get the template system to work the way that I thought that it should.

I’ve got my content type registered as ‘vehicle’ using:
register_post_type ("vehicle", $args);

I’ve tried to get the main listing page set up using a template file archive-vehicle.php but this file is not used. It keeps going back and using the standard archive.php file. From what I’ve read about the template system, shouldn’t it use a file archive-vehicle.php before archive.php or am I doing something else wrong?

If there’s any info that I’ve left out please let me know. I’m only new to the forums here, so not quite sure of the level of detail that’s required sometimes.