How can you know you're getting the best price on your rental car? With a dozen companies offering countless special rates, no customer could keep track of it all. Enter, the black-magic method. The site finds thousands of coupon codes that may apply to you and your rental, so you don't have to do the work.

I made a refundable reservation for a midsize car over a four-day weekend in Dallas-Fort Worth. The best deal I could find at DFW airport three weeks in advance was $136.41 from So I entered my reservation details with AutoSlash, along with information on my various rewards memberships.

Within an hour, I got an e-mail saying "AutoSlash found rates as low as $116.15." Here's the sweet part: This particular rate was available only by specifically booking Alamo through — which never would have occurred to me — using corporate ID 7016141 and coupon code AF5592FDN. Those exact codes are not important — the point is that AutoSlash found the specific alpha­numeric combinations I could not. I booked the new rate on Priceline and canceled my original reservation.

Ten days later, I noticed that rental rates at DFW had fallen across the board, so I went through the AutoSlash process again, dropping to $109.75 with Alamo and then to $92.39 via the Priceline-Alamo "trick." And four days before my trip, AutoSlash e-mailed me with yet another nice new low — $87.09. With little effort, I'd saved 36 percent.

Simon Peter Groebner