I have found, using the same method, and comparing to sat nav that the speedos in the last few cars I have owned have all been 3mph out, not a %age. I was also told at a speed awareness course that you will never be fined if exceeding the speed limit by less than 10%. A Traffic officer told me (a few years ago) that Home Office guidelines were 10% + 2mph, but only guidelines. Another thing I learned on a SA course was the old fashioned square Matrix speed limit signs in the central reservation on a motorway are only advisory as they don't have the red circle around then. The newer overhead ones do.Locally we have a couple of those "watch your speed" signs that shows your speed as you go past and there are also a couple when you travel across Dartmoor as well. Since I pass them all quite frequently I have spotted the discrepancy between the speed on the signs compared to the speed on my speedo. Because I am a sad git I decided to do the same test using the sat nav, keeping my speed as steady as I could I got my partner (yes they thought it was sad as well) to watch the speed on the sign while I checked the speedo and the sat nav. The speedo was almost 10% too low (depends on speed, more inaccurate the faster you go) and the sat nav was either the same speed as the sign showed or +/- 1 MPH of the sign. This was the same across 4 different speed signs, 1 set for 20mph, 1 for 30mph and 2 for 40mph. So while it is far from scientific I thought it was a pretty good way to test.