note the reason you don't admit to buying in cash is because the salesperson makes commission off the amount of money you'd be expected to pay over the life of the loan, including the gargantuan interest amount, so they're incentivized to give you a much better price on the car knowing that the interest will more than make up the difference and help clinch the sale. if you say "I'm buying cash", no discount.
keep in mind that some purchase contracts are canny to this tactic and will make the discount only apply if you're financing, and the financing will renege on the discount if you pay off your loan within six months or some other fixed timeframe. read contracts carefully. you can still make out like a bandit if you time it carefully and never tell the salesperson what you're planning on doing
