If all you want it for it to let you know your speeding get a sat nav, I use the one on my nokia n8 I have set that so if I'm on a road with a speed limit of 50 mph or less and I go more than 3 mph over or on a road more than 50 mph and go over by more than 6 mph it tells me to slow down and will repeat it's self until I listen.
good way to stop you speeding as it's annoying as f**k.
and it gives a more accurate speed than a car speedo, I do 70 by my sat nav and my car says 79 , I know the sat nav is right as when I'm behind a lorry it reads 56 my car reads 61 and lorries are limited to 56.
and it doesn't matter what you get, nothing will help if a copper follows you like that.