The log records pressure (in psi/bar) and tread depth (in mm or 32nds of an inch) for all four tyres monthly, flagging when tread nears the 1.6mm / 2/32-inch legal minimum in most regions or when pressure drifts more than 10% from the door-jamb placard spec. Checking tyres cold once a month catches slow leaks and uneven wear patterns months before they'd otherwise be noticed.
At a glance
| Metric | Threshold |
|---|---|
| Legal minimum tread depth (most US states/UK) | 1.6mm / 2/32 in |
| Recommended replacement tread depth | 3-4/32 in for safety margin |
| Pressure check frequency | Monthly, when tyres are cold |
| Normal pressure loss | ~1-2 psi per month from natural permeation |
Why pressure and tread need to be logged together
Underinflation is the single biggest driver of premature and uneven tyre wear, so tracking pressure and tread side by side helps you catch cause and effect: if one tyre's tread is wearing faster on the edges, a pressure log going back a few months often reveals it's been running low. Logged separately, that connection is easy to miss.
How to measure tread depth at home
- Use a tread depth gauge (a few dollars at any auto parts store) at three points across the tyre width — inner, center, outer.
- Or use the coin test: insert a coin into the groove; if the top of the raised design is visible, tread is near or below the legal minimum.
- Log the lowest of the three readings, since that's the point closest to failing.
Reading uneven wear patterns
| Wear pattern | Likely cause |
|---|---|
| Wear on both outer edges, center fine | Chronic underinflation |
| Wear in the center only | Chronic overinflation |
| Wear on one edge only | Misalignment or worn suspension component |
| Cupping/scalloping pattern | Worn shocks/struts or unbalanced tyres |
Worked example log
| Month | FL psi | FR psi | RL psi | RR psi | Lowest tread (32nds) |
|---|---|---|---|---|---|
| Jan | 34 | 33 | 32 | 32 | 8/32 |
| Apr | 31 | 33 | 32 | 31 | 7/32 |
| Jul | 27 | 32 | 31 | 31 | 6/32 |
In this example, the front-left tyre is losing pressure faster than the others each quarter — a pattern worth investigating (likely a slow leak from a nail or valve stem issue) rather than just topping it up each time without asking why.
Always check pressure when tyres are cold (car hasn't been driven for at least 3 hours, or less than a mile). Driving raises tyre pressure by 4-6 psi from heat, which can hide a real underinflation problem if checked warm.
When the log tells you it's time to replace
Plan for replacement once any tyre's lowest tread reading hits 4/32 inch, especially before wet-weather seasons — stopping distances on wet roads increase noticeably as tread approaches the legal 2/32 minimum, so waiting until the legal limit leaves little safety margin.
Frequently asked questions
How often should I check tyre pressure?
What's the legal minimum tread depth?
Why is one tyre losing pressure faster than the others?
Does tyre pressure change with the seasons?
Sources & further reading
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