World's Fastest Diamond Dust
Why not measure the speed of your plane and fly it through a speed trap and video tape the flight(s). From the tape, you could time the pass repetitively (Rewind/Play, repeat) to get a good average time. Distance over time = velocity
When filming, be sure to shoot from midcourse and a fair distance away (helps camera person track the subject, and reduces parallax). The course should be well marked (so it shows up in the image) and the plane’s a contrasting color from the background (so it shows up too).
Don't bother to convert to PC video capture. Unless you can guarantee 1) the frame rate of the capture is exact, and 2) no frames get dropped and 3) you haven't a berated your time, then don't rely on this method. I have seen some exaggerated results as a result of insufficient controls over the capture/conversion process already.