While coins can be used to estimate tread depth, using a calibrated tire tread depth gauge is a far more precise way of knowing how much tread you have left, and when to start budgeting for a new set!
In the U.S., tread depth gauges measure in inches, specifically in 32nds of an inch, while countries using the metric system, measure in millimeters. Typical U.S. tire tread depth gauges measure up to 32/32" or 1" (25-26mm), as almost all passenger car and light truck tires begin with less than one inch of tread depth.
Curious how to use a tire tread depth gauge? Follow these steps.
Step 1: Push the tread depth gauge against a flat surface, fully inserting the measuring pin into the barrel as far as it will go. Ensure the gauge reads 0/32 to make sure it "zeros out."
Step 2: Place the measuring pin into the center of the main tread groove and push down on the gauge until the base makes firm contact with the tread on either side of the gauge.
Step 3: Carefully remove the gauge by holding its barrel, without touching the measuring pin, and confirm the tread depth reading.
Step 4: Repeat steps 1-3, at least two more times, ideally 15 inches apart.
Step 5: Average all readings.
Step 6: Identify the remaining percentage of tire wear by comparing the tire's original, new tread depth in its specs to the remaining tread depth as measured.
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Note: Incorrect air pressure, vehicle misalignment, loose or worn parts, driving conditions, load, etc, can cause rapid or uneven tire wear. An accurate tread depth gauge can detect developing wear patterns earlier than simply looking at the tread. This often allows the cause to be identified and corrected before excessive or irregular tread wear ruins the tire.
PS: If you're still bent on using a coin or you don't have a gauge handy:
The top of Lincoln's head to the edge of a penny measures about 2/32" - the legal minimum in most states
The top of Washington's head to the edge of a quarter measures about 4/32"
The steps of the Lincoln Memorial to the edge of a penny measures about 6/32"
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