Is it possible / safe to use winter tyres of 195/50R15 on a golf currently fitted with 205/55R16? My son changed his car and tells me if he gets his winter tyres fitted to different rims because if the diameter size it'll be fine. I'm a typically worried mum and would appreciate any advice. Thank you!
This is 100% safe and in fact it is quite common to people (including my self) to use smaller set of rims in the winter.
The reason why many people do it because the smaller diameter tires are usually cheaper.
The difference in tire diameter causes the speedometer reading to be inaccurate by about 9% in this case. Meaning, with the smaller tire, if the speedometer is showing 60Mph, the car is acutally going at 54.5Mph. So your son is actually driving slower than what he thinks he is driving at.
More information can be found at: https://tiresize.com/speedometer-calibration/