Off Topic but:
Saying watching the speedo is a cop out, this is not the only indication of your speed, anyone who is a half decent driver and had a good feel for their car will know what speed they are doing and if they have deviated from that by feel, from here the occasional glance is all that is needed, if drivers are not capable of this then maybe they need to hand in their licence and catch the bus.
I agree that you get a good feel for speed over time, but I also don't think it's realistic to be 100% accurate in every moment. You're always going to fluctuate slightly as gears change and the road undulates. That moment that you get the feeling you need to glance at your speedo might be the one in which you drive past a camera, and then it's too late, as I discovered yesterday.