Your wife should be able to create a new account, you could share the vehicle, and then you can both fuel-up with different cell numbers. To be honest I haven't tested this, but it should work. If you want to be a guinea pig and help us out you could try it.
Hello, I share a car on here with my wife, she entered a fuel up via SMS and I have a icon to edit that fuel up but i get this message when i try. Her account does not show a icon to edit fuel mileage but i think that is by design, correct?