Having checked the car speedometer reading against my sat nav gps reading I believe that the speedometer is under reading across the range. At an indicated 30 the gps shows 34 and at 60 it shows 63. Even allowing for a bit of leeway as the speedometer is way off to the left of my normal line of vision its still not right, as evidenced by the roadside 30 limit warning signs illuminating when I drive in to a 30 limit under an indicated 30mph which doesn't happen in my other car. Looking at the VDO website it looks relatively straightforward to recalibrate however I wonder if anyone knows the answer to the following two questions not answered on the website.
1.In order to change the pulse per mile calibration is it simply a case of taking the percentage difference and applying that to manually calibrate the speedometer? I have a strange feeling that as the calibration range is between 500 to 399,999 it won't be as simple as that.
2. If I can't manually calibrate then the auto calibration option will be the one to go for but where are there reliable mile marker posts?
Any help gratefully received as I have a clean license and would like to keep it that way but still remember getting two speeding fines in a week, one in a car the other on a motorbike, and don't want to repeat that if possible.
