ORIGINAL: tonykalil
Hello SD2006,
Your math is just a little off. Looking at the video again, it looks like the gun is about 12-15 degrees to the side. He is getting the top speed reading when the car is less than 20 feet in front of him. At 15 degrees, you are only getting about 95% of your total speed. So dividing 71 by .95 gives you 74.73, or 75mph. At 12 degree, you are about 97% accurate, but that still gives you 73.19.
I'm pretty comfortable with trig and I get the same % errors you do, but the 6 MPH error you mentioned in a previous post wouldn't be caused by Cosine error unless your car was going insanely fast or your gun angle was a lot more than 10 degrees.