I have no chance of knowing if anyone has messed with the stem. There are no papers followed the car though, indicating any possible repairs in that area. I was thinkning about your guess with the tires. When I bought the car it was fitted with 16" tires. My guess is that since the car is a 300SL it has been fitted with bigger tires and I thought that the speedometer was calibrated with those tires. Could that be the case?? In my mind that would result in the speedometer showing a lower speed. I don't know , , Maybe I way off?