In my '06 4.8iS the 20" staggered tires are standard.

The speedometer is reading 4229 impulses (from speed sensor, that might be equal to tire rotations) per one kilometer, while the odometer is registering 4282 impulses per one kilometer.

those values are specified in the FSW file but I do not know what would happen if you change those values... It seems that the 4282 impulses are the correct values to represent the speed but, as the folks have mentioned before me, the speedometer is deliberately calibrated to show higher then real speed. the digital speedometer shows the correct speed in km/h...