Keywords in the URL is part of the algorithms, so I would suggest it if it is not used in a spammy way, which you are not.
I might suggest these URL's: placethatsellsusedcars.com/dover/used-fords or placethatsellsusedcars.com/used-fords/dover
I suggest this because it will benefit your website's architecture. It will give you website a good flow and organization. Use the one makes most sense for your website. Be sure that the /dover or /used-ford pages have links to all of the next level pages.