AVIC Z140BH GPS location very inaccurate

[blue03si]

When I use GPS my location is always off from where I actually am. If I'm no a route it usually snaps to the route at some point, but it usually tells me I've reached my destination before I actually have and once I reach my destination it shows me down the street from it. My GPS antenna is mounted under my dash and usually has pretty good signal, could that still be the problem or might it be something else?

Thanks!