Originally Posted by Dackelone
Its not only the tire tread depth in one spot on the tire. I think you have to have THREE tire ribs(channels) with at least your states minimum tread depth. Do you have that penny depth across three thread ribs or channel/groves?
You could contact your local State Police office. They are the ones who certify your local inspectors. Ask them what the law is... they will tell you.
15,400 miles is not too short of tire life - on a BMW with a sports suspension. Some only get ~8K miles. Some get more than 20K. Just depends on how you drive. Basically its normal to replace the rear tires TWICE for every front tire set. Just buy to rear tires and call it a day. You really do not want to be driving around on worn out tires like that when it rains or snow.
This is true. Don't expect high mileage from Z-rated tires - getting 15K+ from them is actually pretty good. The rubber on these tires is softer than non-Z-rated tires, and unless you buy some of the newer designs with assymetrical tread compounds with higher UTQG ratings, you generally won't see more tread life than what you're currently getting.