tire tread depth gauge is a tool used to accurately measure the
depth of tread on a vehicles tire.
Tire tread depth gauges are usually either feature either digital
or mechanical readouts. The most common and by far the cheapest
is the mechanical style depth gauge. These usually consist of a
small, pen-like device with a central plunger which can be depressed
to provide an accurate measurement from the top of the tire tread
down into the grooves.