Hi there,
Unfortunately, car seat manufacturers must put a height restriction on their carseats by law in Canada. It is unfortunate because a more accurate measurement would be a TORSO height restriction. There are some kids that are all legs. Mine is all torso (4t shirts, 2t pants
), and outgrows his seats more quickly than the height restriction would indicate.
Although as a technician, I cannot tell you to go against manufacturer's instructions, I can tell you that a car seat is outgrown rear facing when they reach the manufacturer's weight limit, or when their head is less than 1" below the top of the shell. The rest is a parental decision.
I can also tell you that as a tech, I have no idea how tall my child is in cms... but I know he fits based on this rule.
You are right that he is safest riding rear facing as long as possible. If it looks like he might be a tall skinny kid, you could look into the First Years True Fit, it does only have a 35lbs weight limit, but it has a nice high shell on it and will fit most tall skinny kids for years rear facing and then more forward facing.