In one of the odder tactics at taking a look at how much money better mileage cars can save you syndicated writer Scott Burns has come up with a formula that shows the real money saved by trading in gas guzzlers for more efficient vehicles. Basically trading in Scott's VW Turbo Beetle for a Prius and he saves $1,000 a year. He then states that earn that much money in dividends on the stock market, S&P 500 specifically, you'd need to have over $70,000 invested. Damn. That makes the Prius worth those extra few bones. But it's not just a one car calculator. Burns has figured out the difference in any two cars. So see if going more fuel efficient is really worth it.