What is the expect range of the 2nd Gen motion sensor?

My motion sensor is installed at 7.5 ft in the back corner my detached garage (dimensions of 20' x 25') and takes about 15 seconds to notice that I have entered the garage before it triggers the alarm base station. If I walk slowly enough it will never detect I have entered the garage for the entire time I am there, even though I entered and exited through one of the overhead doors. Is the garage too big for the range of this sensor? As a basis of comparison, the hardwired Honeywell motion sensor mounted in the entrance hall of my house is able to detect motion at 50' away from the entrance hall to the back corner of my great room. Likewise the one mounted in the opposing direction has the same ability.