Up for sale is a NEW (Tyre) tire Pressure Gauge / Tread Depth Gauge with flexible stainless steel braided hose and protective rubber boot. Reads up to 120 PSI. and has a neat tread depth gauge on it as well.
The depth that a tire’s tread starts out varies depending on the type and brand of tire, but all tires needs to be replaced when they get to 2/32nds of an inch. You should probably start planning for new tires when your tread is down to 4/32nds of an inch.