Who is liable for a summons for car a wife got in divorce both on title though?

It doesn't matter who is on title, what matters is who is on the actual note (which is not always the same person or people). If she got the car in the divorce settlement, but you are on the loan and/or both of you are on the loan, the Lender is not doing anything improper by suing you. That being said, if you had an actual separation agreement or court order detailing how liabilities would be split, there may be what is called "indemnification" language in there for this very scenario and/or details in there that she was supposed to refinance the loan into her own name but never did. You should consult with an attorney ASAP to review all of your relevant divorce settlement documents and the recent summons because there is typically only 30 days to respond to that.