If you just want a stand-alone unit, it will be inaccurate and counter-intuitive to your goal of being accurate to avoid passing the speed limit. Add-on speedometers rely on GPS or accelerometers which lag behind and are generally inaccurate compared to the actual sensors for your wheel speed. If you can tap into your car's sensors, that is your best bet.
You could try something like this if you can tap into your on-board diagnostics system and will reflect off your windshield.
http://www.dx.com/p/a8-5-5-screen-hu...5#.VfxSXBFVhBc
http://www.dx.com/p/w02-5-5-hud-head...6#.VfxRyhFVhBc