Upvote:0
No, such a tool does not exist as of 2018.
Upvote:3
I don't think it exists, and probably because there is little need / support.
Car rental prices are typically constants per day and per week, with the price for the week being about five times the daily price. With that, it is relatively easy to do the calculations - shifting the start day has zero effect, and increasing the length gives every sixth and seventh day no or minimal price change.
On top of that are of course some company specific 'specials', those have all kind of rules that change all the time, and they are typically limited in duration. They are not always known in advance, so it is difficult to predict them.
So a matrix comparison would be often of little interest, and the specials are not accessible in any automated way (like for flights). Also, most people are bound with their dates from the flights they book.