How much below asking price should I offer on a car?

There's no universal percentage. A reasonable offer depends on how the price compares with similar vehicles, how long the car has been listed, demand for the model, current incentives, and the vehicle's condition and history. Base your number on comparable out-the-door prices you've found and be ready to explain it. OffLot shows days on market, price changes and MSRP (for new) in each listing's facts to help you research, without labeling any listing a good or bad deal.