Question between my gf and I?

So I want to rent a car (nothing fancy) and do Uber , Lyft, maybe postmates or DoorDash but don’t wanna put miles on my own car. He thinks this will leave me making no money (renting the car) so we are at a disagreement. Uber recommends renting through Avis so I proposed we try it for a week. At first he was ok with it, now he wants to see the numbers of what id make and it would cost etc but i feel there’s no exact science to what id make - id put a goal # on each day and some days have to work more and some less to reach the goal. What do i do?