To get an information about average or maximum speed I included a digital speedometer Sigma BC 1200. For calibration it is necessary to input the exact wheel circumference which is also called wheel size factor. It can be measured on the car or it can be determined by the help of this table:
The sensor I use is a wheel sensor with magnet glued on the inner side of the rim. If a cardan-drive shaft sensor is used, the wheel circumference must be multiplied by 0.2195 to get the wheel size factor.
I calibrated the speedo with a GPS based navigation system. The correct wheel size factor for 195/60 R 15 tires is 1882 mm.
Digital instruments unfortunately are not always allowed in historic rallying and this particular one has not backlight function.