Posted for "Ed Anderson" <eanderson@carolina.rr.com>:


     Right, Ian

         My performance figures are true airspeed.  GPS will provide Groundspeed, but not true airspeed (unless there is no wind or you use the GPS on different heading to determine windspeed component and correct for that).  TAS is a better measure than IAS in that desensity altitude and instrument error is taken into consideration.  IAS may be 10-15 knots different than TAS.         I know of one case. a fellow rotary powered RV reported achieving over 200 mph when I was only making 186 MPH - made me feel puny {:>).  Later turns out he had a large instrument error and his aircraft was  actually doing around 186 MPH rather than 200.

        So until you calibrate your IAS for instrument error and use density altitude (Temperature and Pressure altitude factors) its difficult to compare.
      FWIW

     Ed