I know there are comparison rental tools like Kayak and Expedia, but I'm specifically thinking of a tool similar to Google Flight's flexible date matrix, where they'll show you the lowest prices for a matrix of different combinations of start and end dates.
I've found that with car rentals, the prices can change drastically just by booking a day earlier or later, or sometimes even just hours apart, to the tune of 50% price reductions, and doing this all manually is pretty time-consuming and error-prone.
To be clear, since this was closed out as a duplicate the last time I asked: I know car rental price comparison tools exist, but those only compare prices between agencies for a specific start-end date combination. I'm asking about whether something similar to Google Flight's flexible date tool exists, where you can get a sense for price changes across multiple start/end dates/times and durations, like so: