The police set up a radar sign near my house so I took advantage of the opportunity to compare the speedometer on my C124s with my measured radar speed. The road has a 30mph speed limit and goes past the police station so I didn't want go too fast but I was able to drive by the sign at 30, 35, 40 and 45 mph. My speedometer seemed to consistently read 2 mph over the the radar reading. I then drove my car past the radar at 30mph for a comparison and both my car speedometer and the radar sign read 30mph.
I consider 2mph to be very close so I am quite pleased with the accuracy of the speedometer. My XM-3000 speedometer was so far off that it was completely useless for indicating speed. If a speedometer isn't exactly dead on accurate. I'd rather it read slightly over than slightly under so I'm never exceeding the speed limit by mistake.