1. Measure the gear's diameter. For this example, imagine that a gear's diameter is 8 inches. 2. Divide the number of teeth by the gear's diameter. For instance, if the gear has 49
http://www.ehow.com/how_8666790_measurepitchgear...

Usually with a 4' socket wrench, but i like to use a crescent wrench myself.
http://wiki.answers.com/Q/How_do_you_measure_diame...

Just to be sure this is what you want, the diametral pitch is defined as the ratio of the number of teeth to the pitch diameter  it's the number of teeth per inch of pitch diameter
http://uk.answers.yahoo.com/question/index?qid=200...

Spectrogram is what measures the pitch and amplitude and graphs it
http://www.chacha.com/question/whatisthemeasure...
