Tire Depth Gauge?

I was fleet maintenance officer for our dept's cruisers and always carried a tread gauge in my pocket. I was one of the worst to wear out tires as I had a reputation for never getting out of my car while on the job unless something happened. I also used the Lincoln Penny which works just as well. The reason we kept an eye on the tread is because we needed handling at all times and never worried about the sidewalls because the tread wore out long before the sidewalls.