How accurate is your speedo

Mine's pretty accurate. It says zero!

 

Mine over reads by around 6 or 7% on stock tyres. eg about 3 mph at 50.

Sat nav is accurate on the level. On hills it will under read because it measures distance in only two dimensions.

 

According to the satnav, ours is exactly 10% out. My excitement at hitting 60mph on the M6 was dampened a bit by that discovery.

 

According to my sat nav and iPhone app my speedo is between 7-10MPH too fast. So when it reads 70MPH I'm actually only doing 60-63 MPH. I'm guessing that's due to the 1600 box on a two litre engine? Or am I being stupid and is only down to tyres and speedo allowance?

 

I have 185/65 R14 86T (I know the rating is a bit low) shouldn't make that much difference should it?

 

So what is the original tyre size? I can only find 185R14

 

80?! That's huge.