You will hear advocates and others say never finance through the dealership. Follow that advice and you could overpay by thousands of dollars. Dealerships often have the very best interest rates.
Dealerships 'buy' loans in bulk and just like any wholesaler gets a discount. That savings can be passed along to the customer if the dealership wants to sell that car bad enough, if you negotiate hard enough, if you just know how to shop and compare.
You get the best deal when you go to your bank or credit union and ask for a quote for a loan in the amount of the car you want to buy. Don't be vague. Exact cost of the car/license/registration and how much you are going to put down.
Now when shopping give the dealership a chance to beat your prearranged interest rate. Sometimes they can, sometimes they can't, but you will know you got the very best deal.