Airguide can be calibrated (using a GPS or stopwatch against a known distance for example a slalom course). Others can't

Imagine using the above two gauges for the following exercises

• 12mph kids kneeboard/surf/disc etc
• Riding at exactly 21mph (note the gauge on the left has graduations of 2mph from 20 to 30mph - i.e. the line after 20 is 22mph not 21)
  
• Slalom at 36mph seems fine on the gauge on the left but it can't be calibrated so your driver must compensate for whatever the correct speed really is
  
• But barefoot at 40mph is great on both (if the standard gauge happens to read accurately i.e. is 40mph really 40mph?)
• Can you think of a speed you or your crew ski at that the airguide doesn't have covered???