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A few months back I told you about my favorite way to save money on hotels. Short version: If you book a stay and the service finds a lower rate before you arrive, you can rebook and pocket the difference.
Too bad there's nothing similar for air travel. But it turns out there is a money-saving service like that for car rentals: AutoSlash.
This site takes a two-pronged approach to helping you get cheap cars: automatically finding coupons and discount codes for your rentals, then tracking those rentals to watch for cheaper rates.
I haven't used AutoSlash myself, so I can't say firsthand if it works as well as, say, Pruvo (the aforementioned hotel-rebooking service, which I continue to love.)
However, I can tell you it takes a bit more work to get it to track an existing rental booking. You can't just forward a booking-confirmation email; rather, you have to manually enter all the information, including your name, the confirmation number, the pickup date, the price you paid for the reservation and so on.
You also have to indicate what kinds of credit cards you have (no numbers required, though), associations you belong to and frequent-flier or -renter memberships you have (if any). In theory, that's so AutoSlash can really plumb the depths of available discounts.
It does, however, feel like you're revealing more personal info than you have to with other price-tracking sites. You'll have to decide if you're comfortable with it.
If you are, this could potentially save a business some considerable car-rental coin. I'd just like to see it get a bit more automated.