Speedometer out by 3miles

So, my speedo reads 3 miles (or in my case, 5km) over what I am actually doing. My GPS and one of those silly signs that tells you the speed you're going confirm this.

Is there any way of fixing it, or am I better just leaving it? It's a bit annoying not knowing what speed I am actually going.

I have an '05 V6 manual tib FWIW

Thanks in advance.