Thanks genghis
The racetrack method I am familiar with, and yes I agree with the pitot / static tests are important. The issue is in a good or bad system, the system does not change. its onlt the variation in result that has me curious.
The low performance bit..... well I would have thought the high performance military jets for example would hardly notice any effect while a 160 knot machine will notice 3-4 knots difference.
Your comments about running along or slightly across the isobars is food for thought indeed.
I will keep an eye on it.....and if I remember to do it, I will post back here.