Since speed is related to dynamic pressure, then why is static pressure used instead of dynamic pressure in ASI to measure speed?
Since speed is related to dynamic pressure, then why is static pressure used instead of dynamic pressure in ASI to measure speed?


It's not. The reading on the airspeed indicator is directly determined by the dynamic pressure. The ASI uses the static pressure for the sole purpose of determining the dynamic pressure. 


  Why is dynamic (or pitot) pressure not sufficient?


Dynamic pressure is sufficient, but the pitot tube doesn't give you the dynamic pressure; it gives you the total pressure, which is the sum of static pressure and dynamic pressure. In order to determine the dynamic pressure, an airspeed indicator has to find the difference between static pressure (from the static tube) and total pressure (from the pitot tube).