They'll believe you & charge you. However, if they go to clean the car & it's not a full tank of gas, they'll amend the total to charge you the appropriate amount.

Several times I've had a car & when I go in to the desk and give the agent my mileage (small apts), they say that's less than when I rented it. They'll page someone in the parking lot to verify I'm right. The person before me must have inverted some numbers or something. No biggie b/c most all non-1 way rentals are unlimited mileage.