You have the right to have the dealer disclose to you in writing, ALL of the finance terms of your purchase BEFORE you agree to buy a car – and then you are able to walk out with the written disclosure and compare financing rates. Then you may return to purchase the car.

The Truth in Lending Act

15 USC §1601

(a) Informed use of credit The Congress finds that economic stabilization would be enhanced and the competition among the various financial institutions and other firms engaged in the extension of consumer credit would be strengthened by the informed use of credit. The informed use of credit results from an awareness of the cost thereof by consumers. It is the purpose of this subchapter to assure a meaningful disclosure of credit terms so that the consumer will be able to compare more readily the various credit terms available to him….