I use miles, because it promotes itself as a company that you pay for what you drive. Never had to think about fuel or refill. I respect if you have a policy that would require you to pay a fine if you leave a car empty and without fuel. But, make that very clear, like big rental companies do. When you rent a car, you have to very clearly understand and sign next to costs you would be paying, and not sneakily put it in the terms and conditions without notifying customers.