What other queries do you do against these tables? Because i do not see any reason of having separate ids for every car number.
Regarding delete and re-insert. You can try to add a columnt last, and do a simple update of records setting for new records last to 1 and for old to 0. If you have more records, then you had, then switch to insert mode. I do not see benefits of this approach, however it works in some of my systems.