No one can evaluate your damages or the chances of a successful lawsuit based upon 5 lines of explanation and you wouldn't want to rely on that analysis if they did. You need to consult with an attorney who is experienced in business litigation. He/she will need a lot more facts including an analysis of the documents involved in order to form any kind of opinion. This transaction occurred in February. One of the questions is going to be "Why did you wait so long to do anything?"
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.
The monthly payment is not much higher than what you were promised, but what about the length of the loan? The Truth in Lending disclosure forms will show how much in total Finance Charges you agreed to pay versus how much you are now being required to pay. This may be a starting point in your decision whether to make an issue of the dealer's conduct or not.
If so, I suggest you contact an attorney with expertise in consumer credit and automobile fraud cases. It's a good bet that whatever the dealer did to you, it did to other customers as well. A lawyer who practices in the field of consumer protection will know what to ask to determine whether the case is worth pursuing. You can get the names of local lawyers from the website of the National Association of Consumer Advocates, www.naca.net.
Disclaimer: This site exists to provide information only. It is not legal advice. Answering your question does not create an attorney-client relationship. I am a Massachusetts lawyer. Any information provided on this site does not, except as explicitly stated, imply familiarity with laws or procedures peculiar to your state which may differ from those where I practice.