My understanding is that all speed and distance recordings are done off the wheel speed sensors. Providing that all the wheels have the same (give or take a bit for tyre wear) circumference, then all should be happy. A larger circumference will cause the speed sensors to under read against reality, but the van has no way of knowing what reality is. It bases its calculations on the information it receives. So if you set the CC at what the van believes to be 50mph, it has no reason to assume it's doing anything other than that.