What led to this experimentation was an ASI labeled in knots but calibrated in miler per hour. It was a contributing factor to a very hard landing while flying at heavier than usual gross weight and a forward center of gravity. In short, the canard of the Cozy   stalled during landing flare. I thought I was flying at 80 knots but it was 80 mph.

After discovering the calibration error, the ASI was recalibrated on the bench to read within 1 or 2 knots at speeds up to 190 knots. But what happens in-flight? Air is moving  and static pressure is not the same as still air pressure on the bench.

The ASI will show a higher reading than on the bench because of less static pressure on the back side of the gauge. Also, the altimeter will show a higher altitude than true altitude for the same reason.

In our airplanes this is no big deal, unless one is very concerned about accuracy, because we do not fly fast enough to cause a great pressure drop. I calculate my ASI is off about 7-8 knots at cruise and perhaps 2-3 at pattern altitude. The altimeter may read 200' feet high at cruise, probably less than 100'high at pattern speeds. This can be compensated  for by installing a fence device near the static port. Some production airplanes have such devices.

Less than reputable individuals will do this not to correct the error, but to make the ASI read faster yet in an effort to deceive a buyer into thinking this airplane is REALLY fast. I discovered a small fence, 3/16" thick in front of the static port, will increase the ASI by 10 knots at cruise. Conversely, the same fence installed aft of the port caused a drop in  speed of some 40 knots. In fact, I couldn't get the indicator about 100 knots no matter how fast I went.

I do believe though, a properly shaped fence installed at the right spot would remove most pressure error but it would take much testing and it probably would not work from one airplane to another.

In high performance aircraft, Air Data Computers are used to apply the error factor to the ASI and altimeter. In these airplanes, the ASI and altimeter are electric/pressure instruments. If electric power is lost, they revert to standard pressure instruments or in the case of all electric instruments, a back up pressure gauge system must be used.

In our small airplanes, the only way we know for sure how fast we are traveling is by GPS ground speed. A three leg run will give the necessary speeds and courses to insert into  any one of several programs running on Excel or internet Applets, and it will come up with an accurate true airspeed. And if temperature and pressure altitude are known, it is easy to come up with calibrated airspeed. And when that number is compared to your ASI, you'll know the truth of your indicator.