Moving radar units do not rely on the car's speedometer. They compute the unit's speed itself along with the difference between the target. And so NHTSA says "don't use a radar if it's not with 1 mph for stationary or 2 for moving" but that doesn't say that this is what the accuracy is determined to be, but most units are within a mph either way, and that's not going to mandate a six or ten mile per hour buffer.
You are missing the point that the is not charged at 40 MPH. He is charged at 50 in what appears to be (based on the poster's statement) of either a 45 or 50 zone. That's way more than is required for even the NHTSA-compliant radar units or even pacing require (we're talking about 11% over the limit).

