What in the highest legal interest rate for a car loan in Washington state?

There is probably no "enforceable" cap on interest rates. If you have ever seen the commercials for some "bluish colored sky financing place", they can get around rate caps because they are located on tribal land and not subject to some laws and exempt from others, and charge something like 8 grand to borrow a few thousand dollars.

As with everything else; buyer beware. Join a credit union and don't get ripped off by shady loans.