I compared my satnav against my speedometer and found it to be 5mph different. Unfortunately i dont have access to police speed checking devices unless i want to pay a fine. Come to think of it i cant remember how i worked out my speedometer was 2mph out. I do remember that it was about that though. Obviously it is just "about" i dont have a way of testing things accurately.

My satnav is usually connected to about 5 satellites at a time but i have noticed that even a little bad weather can cause problems. I dont think its that reliable and my satnav is supposed to be one of the best ones out.

Why exactly do you need something thats more accurate than a speedometer? I can understand if you would rather see a number instead of a dial but why go to the extreme of GPS?

I understand that wheel size and tyre pressure can affect your mph but i thought you should have it callibrated for different wheel sizes. as for tyre pressure i dont think it will affect it that much. Will your display have a decimal point? Can it display "10.54mph"? Unless you have a decimal point then i dont think it would make a difference