I have noticed that when I look at the speed on the Sat Nav it is always slower than the speed on the Speedo. Then, when passing speed signs in certain villages (the ones that show the correct speed or lower in green but red when you go over the limit) that they read the same as my Sat Nav. Which is more accurate? My son, who is a mechanic, says that the Sat Nav is more correct because it is based on actual speed and distance travelled. If that is the case then my MPG is actually higher than I thought by more than 10%. I wonder is this is because the speedo is for all models no matter what size wheels you have. Any of you boffins know the answer to this?