If you really want to know how fast your plane is going, you're going to have to do a lot of work to insure that your instrumentation is correct. One of the first big errors is your OAT. When you're in the 200 mph arena, indicated OAT is not true OAT, if your sensor is in the airflow. Stagnation rise is about 8 dF. That will throw off your dalt 500', which will give about 0.8% error, or 1.6 mph at 8000' dalt. If you are using a Dynon EFIS, they don't correct for stagnation rise, so you need your OAT sensor out of the flow, say back in the tailcone. Other avionics use various percentages of the stagnation rise. If you use your Garmin's E6B to get TAS and dalt, they want total temperature, static plus stagnation. The way to tell if your OAT is giving you correct numbers, is, after climbing to some suitable altitude, slow down to about 80-90 mph TAS and note your OAT and, if available, your dalt. Now get up to high cruise, and again note these two and compare them to the previous values. 'Bet they're not the same! BTW, you can use two-way averaging of GPS GROUNDTRACK numbers if the wind is less than 20 mph and you're going about 200 mph to obtain TAS to find your errors, both from OAT and static port errors. 'Doesn't matter which way the wind is blowing as long as it's the same in both 180 deg different GROUNDTRACK directions. Your error will be less than 1 mph with the wind 90 deg. If you use preflight winds-aloft estimates, your error will be masked by your ability to hold IAS steady.
