Whether or not the dealer is a single line is pretty much up to the dealer principal.
Our organization keeps most (if not all) of our franchises in separate locations and each facility is a single carline.
Personally I prefer it this way, it lets me concentrate on fixing one brand of car instead of trying to keep up with 5 brands of automobile. It is better I think to know 5 cars from bumper to bumper than it is to know a little bit about 50 of them.
It lets me take better care of my customers.
The last time GM tried to inflict branding regulations on dealers it caused a huge mess and most dealers ignored their threats anyway. It didn't work out at all and the places that changed to conform usually ended up having some issues and losing a lot of money.
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