System on the current horse computes TAS from the ADC and COG and GS from the GPS to bring up a wind barb on the EHSI just like you would see on a weather map. The ADC just sticks it in the GPS automatically most still have a manual option if you have the time or inclination.
Plus or minus 5 knots I would say and of course once you slow down it is not so accurate and will drop out below a certain speed but is close enough for government work!
Then again as I was told many moons ago as a sprog if you don't know where the wind is coming from and how hard - all the time - you shouldn't be in the drivers seat. There are a plethora of cues/clues available.