Tread depth and tire pressure checks

A tread gauge is a cheap tool that answers one question exactly, and the threshold is not a matter of opinion. The National Highway Traffic Safety Administration is explicit: a tire is not safe and should be replaced once the tread is worn down to 2/32 of an inch.

Three ways to read the same thing

• A depth gauge. Direct reading in 32nds. Take it across several points on each tire, not one, because wear is rarely even.
• The moulded indicators. Tires carry raised treadwear bars between the tread blocks. When the surrounding tread has worn level with those bars, the tire is finished.
• A penny. Insert it with Lincoln's head upside down and facing you. If the top of his head stays visible, the tread is too shallow and the tire needs replacing.

NHTSA suggests checking tread at least monthly, at the same time as pressure.

The pressure routine that gives a true reading

• Use the figure on the vehicle's door placard or in the manual, not the number moulded into the tire sidewall. The sidewall figure is a maximum, not a recommendation.
• Read pressures cold, meaning the car has stood at least three hours. A warm tire reads high.
• Check the spare as well as the four on the ground.

There are 2 tread and depth gauges in this range, which is the sort of thing worth keeping in the glovebox rather than buying when a tire already looks worn.