That is a possibility too. On a mass-produced car, I'd evaluate this as pretty low probability, since setting speedometer for the actual car specs is a basic expectation and if any company would let wrongly calibrated cars out of their factory it would be a huge liability issue and a very loud scandal. However, if custom modifications were made to the car, it is not out of the question, theoretically. But then the effect of the smaller tires should be the opposite, not? I.e. with smaller tires the speedometer should overestimate the speed, not underestimate?