Does doubling up car payments save money?

Right now i am saving up for a new tiburon approx a 20,000 car.

anyways, plan is 14,000 cash downpayment by september.. finance 6,000 over 1 year or less.

But have question, on double up payments.
When u take out a loan for a car... isn't the total cost fixed.
so if u doubled up payments it woudl have no effect on how much u eventually pay.

In other words.. The interest on a double payment, would be twice as large as on one payment right? meaning in the end even if u pay triple or quadruple payments, u'll still pay the same amount of interest.
Meaning u really save nothing by making larger payments..

and thus a bigger downpayment is by far the best choice finance wise?