This Basic Stamp Microcontroller Based Garage Parking Assistant may be overkill for the application but that is what makes it fun. What fun would it be to hang a tennis ball from the ceiling when you can use technology to give you a traffic light that represents your optimal parking distance.

"The Garage Parking Assistant makes it easy to pull in the garage by signaling you with a traffic-light style display of when to pull in (GREEN), slow down (YELLOW) and stop (RED). If you pull too far in the Red LED blinks to let you know. Although this type of project has been done before I wanted to take my own approach and simplify the code and hardware while making it easy to adjust/personalize for your own use. By default all measurements are in inches, although you could easily use centimeters since that value is available as well. The range of the PING))) sensor is split up into four zones labeled Zone1 through Zone4. Please see the first attachment for a map of the zones. The values for each zone are inches from the sensor/wall."