This is a loaded question i have been asked not hundred of times through the years but many thousands times and it really branches out with many different answers, here are some answers the average consumer was not aware off

New cars...as an example lets say a vehicle is $ 49.495. and the dealer says 0% financing, well lets look a little closer at the overall picture that is you have not read the fine print as most consumers dont.

0% - 24 months ( thats great if no money up front and you can afford heavy payments )
3% - 36 months ( nice rate but still heavy payments )
5% - 60 months ( i am starting to like the payments, but what happened to the interest rate )

Buying down Used Car Rate

You have done all your homework and know what interest rate is fair shopping for a car and you come across a dealer offering interest rates a notch or two below average.?
Dealers know how to press that emotion button dealing with smart shoppers shopping interest rates as one of the main factors buying a car.

Don't ask whats your Interest rate ?"- You are telling the salesperson that the interest rate will come into play in your decision making. Some lenders will allow for a dealer to buy down the rate and if a point or two below average triggers your emotions, you are hooked. " example " the dealer pays $800. towards the Interest to the lender up front and the rate now drops 2 points.
The $800. will be hidden in the negotiation stage, in other words had the Interest rate not been a factor to close the deal then there would be $800. more on the table that could be negotiated..

You dont ask the dealer what their Interest rate is before having finalized a price, you have a rate in mind and it now comes time for dealer to match your expectations or you shop for your own financing.