speedometer calibration

First I did a search on this and couldn't find an answer.
I am aware that there is a calibration knob on the speedometers

My two speedometers will not track at the same rate. If I calibrate them at let's say 5mph they are off at 20mph.   I looked up how they work, water pressure to air pressure. So I imagine this means I have an air leak? Where is the best place to start trying to find the leak?
Or am I assuming incorrectly and this is not likely the root cause and it is something in the gauge or maybe the pressure pickup at the back of the boat like some material got in their or their is a filter I can clean?
In summary what is the best sequence for diagnosing?
Thanks,

-1 on what DrCC said.

Make sure that you blow from the speedometer to the pitot and not the other way around.

Several years ago, I used a hand-held GPS unit to verify and calibrate my 2 AirGuide speedometers. Then I set both to read the same at 34 mph (my ski speed). Then I checked to see which of the 2 speedos was the most accurate at 20 mph (our wakeboard speed). Once I did that, I used the designated wakeboard speedo for boarding. I ran with that set up for several years.

Recently, I replaced one of the AirGuide speedos with the Perfect Pass Precision Star GPS speedometer. Love it.

JQ