In the Zenith CH701, the cockpit airspeed gauge does not match the HUD airspeed.
At all speeds, the HUD airspeed is 85-87% of the cockpit gauge airspeed (example: cockpit = 100 / HUD = 87).
I do not know which readout is more accurate. Other aircraft I tested do not have this error.
Is anyone aware of a setting in an aircraft configuration file that will allow me to calibrate the airspeed display?
What unit of measurement does the gauge in the cockpit use? It may be measured differently than on the outside HUD. 
Cheers!
Great suggestion, HamMan 2118. And, yes, I considered the fact that the cockpit gauge is MPH and the HUD is KPH, but forgot to mention it in my original post.
However, the issue is still the same. Conversion of MPH vs KPH uses a factor of 1.609. The discrepancy between the gauges in approx. 0.87.
And the relationship between MPH and KPH is reversed. The cockpit (MPH) should be a smaller number than the HUD (KPH).
I’m hoping this is something I can tweak in a .cfg file.
You have the relationship reversed. 
100mph (cockpit) is equal to 86.9kts (HUD).
This, and it’s subtle in context, so to clarify, I think OP is converting to/from kilometers per hour, when the conversion should be, as you said, knots (nautical miles per hour) to (statute) miles per hour.
1 knot = 1.15 miles per hour
1 mile per hour = 0.87 knots
Thanks, CharlieFox00. That nailed it. I was misreading the HUD gauge. It is indeed knots and not kilometers. Now that I am looking at it correctly, it now makes sense.
I appreciate everyone’s fast response. Thanks!
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