I posted this as a comment under the Gambling with an Edge show on 7/19/18 where I mentioned autoslash, but thought I would post it here as well.
In the past, I have recommended booking a rental car with one of the websites for rental cars (carrentals.com, expedia.com, Costco.com) and then putting that information into autoslash.com. It has been my experience that this method produced better results than booking the original rental directly with autoslash.
I don’t know if autoslash.com made changes to their booking tool or I was just an unfortunate outlier, but I did some testing and booking directly with autoslash.com will now produce the lowest rate available. I will test it a few more times before I totally abandon every other rental car site but I am pretty confident that booking directly with autoslash.com will produce the best results.
This method now saves you even more time. Go to autoslash.com, punch in your travel request and let them do the rest. They will check rates up until your pick up date to find you the lowest fare. It’s a great service and the price is right. Free.

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I have also found AutoSlash to be the best of the car rental providers, normally by a large margin.