As a follow up, I mounted two of these ultrasonic sensors on my double garage ceiling using heavy duty hook and loop strips. They are working quite well for my purposes.
I originally tried to precisely measure the distance between the sensors and the garage floor, and got pretty close. But after I activated the sensors, I realized that they themselves could determine their own distance by whether the nominal value I used resulted in a negative number. For example, I put in 2.9m and both units were registering -0.1m. Accordingly, I adjusted the installation height parameter to 2.91m and they generally register 0m when a car is absent. Note that they actually have a resolution of about half a centimeter.
Last note, in order to increase the WAF, I covered the orange shell of each sensor with white electrical tape. If I get ambitious, I will carefully spray paint them white.
