When I'm driving and someone says something about my speed, I tell 'em to shut up. My dad has gotten to the point where he keeps his mouth shut when he's riding in the front seat b/c he knows that I'll blast him. I put up with his sh!t when he was teaching me to drive, but I don't put up with that crap anymore
When I'm driving with someone else in the car, I never speed excessively (maybe 5-10 over at the most) in the first place so they need to shut up. If they want to drive, then drive. But when I'm beind the wheel, leave me the hell alone and stop side-seat driving.
Center mounted guages are more for style and cost reasons than anything practical.