Re: Wrong question
If you feel the need to watch your speedo constantly to know what speed you're doing, you shouldn't be driving a car.
If you're talking about being able to keep your speed within ± 5km/h without looking, say when sitting at 90km/h on a main road, I'd agree. Having been the victim of a camera-generated ticket for doing 51km/h in a 50 km/h zone, though, I'd have to say that that level of accuracy is likely to be beyond even the best of us. Of course I could have just pottered along at 45 to be sure, but the folks behind me wouldn't have appreciated it.