New cars are not investments, they are liabilities. I think they lose about 40% of their value in the first 3 years. So unless you plan on keeping them forever, new cars are a colossal waste of money.
Gap insurance is a good idea on new cars. If it gets totalled in the first couple years of ownership, chances are you'll be upside down on it if you haven't used a big down payment. It's cheap insurance.
Extended warranties, particularly on cars you only keep for 3-5 years, are a horrible investment and are padded heavily to give the dealer more profit.
Something NOBODY does...consider resale value down the road and balance that with whatever financing or 'deal' you may be getting now. My father had a hard-on for a Taurus some years ago because of 0% financing. I have nothing at all against a Taurus, but I pointed out to him the residual value of a Taurus at 6 years of age compared to an Accord. The value retention of the Honda easily chumped whatever financing costs he would save when he went to sell/trade the car (he typically replaces every 5-6 years). He bought the Honda.