i'm not sure how its done, but when calibrating for speed, you can enter different info into the computer than what the actual numbers are...therefore it will read the mph differently than actual speed.  Sorry, I don't mean to confuse you.  Also, I have NO experience doing this myself, but I had discussions about it with a ASM Specialist.

Trying to explain...when calibrating the machine, you drive 22 feet per second = 15 mph, but when you calibrate the computer you enter that you only went 16 feet per second at that speed, THEN it might allow you to travel 15 mph (that what it thinks you're doing) even though your really going 20.8.

Get it?  Sorry for any confusion....