tire-pressure-coach

Use when checking or setting tire pressure on any vehicle — cold vs hot pressure physics, placard-vs-max myth-busting, load and speed corrections, temperature/altitude seasonal deltas, plus tread-depth safety math with legal limits and wet-grip cliff. Includes TPMS decode and monthly-check scheduler.

Overview

Tires are the only part of a car touching the road, yet pressure is set by guesswork and gas-station air pumps at the wrong temperature. This skill runs the real physics: ideal-gas pressure-vs-temperature deltas (≈1 psi per 10°F / ~0.06 bar per 5°C), cold-tire protocol (3h parked / 1 mi driven rule), placard logic (the door sticker, NOT the sidewall max), load and speed corrections, seasonal changeover math, and tread-depth safety (legal limits vs the wet-grip performance cliff at ~4 mm, penny/quarter tests).

When to Use

• "What pressure should my tires be?" — placard-first logic, corrected for your conditions
• Filling tires that aren't cold (the gas-station problem) — target-pressure math so you don't systematically underfill
• Seasonal swings / road trips to different climate or altitude
• "Are my tires worn out?" — tread depth → verdict + wet braking risk
• TPMS light on: what the threshold means and what to check
• Setting up a monthly check habit with exact psi targets per axle

Don't use for: tire sizing/plus-sizing questions, wheel offset, or tire brand shopping (can advise safety-adjacent context only).

The Physics (what the script computes)

1. Cold target = door-placard value (driver's door jamb, fuel flap, or manual — NEVER the sidewall max, which is a rim limit, not a setting).
2. Temperature correction: pressure scales with absolute temperature. ΔP ≈ P × ΔT/Τ (T in Kelvin). Rule of thumb at ~32 psi: 1 psi per 10°F. Cold morning vs warm afternoon matters; so does an unheated garage.
3. Hot-tire fill protocol: if you must fill warm, fill to placard + (current_tread_temp − ambient assumption) — practically: measured hot pressure ≈ 2–4 psi above cold; the script computes your fill-to number from measured hot reading.
4. Load correction: full load / towing / high-speed (Germany-style) typically +2–4 psi on rear axle (per manufacturer high-load placard values where they exist).
5. Altitude: absolute vs gauge — driving 1500 m up changes gauge reading by ~2 psi; matters on mountain trips.
6. Tread verdict: depth mm → legal limit check (most jurisdictions 1.6 mm; wet-performance cliff ≈ 4 mm; winter-tire practical minimum 4–5 mm) + asymmetric wear flags.

Numbered Workflow

1. Find the placard (driver's door jamb): front/rear psi + size + load rating. placard command records it per vehicle.
2. Check COLD (3+ hours parked or <1 mile driven): check with measured psi and ambient temperature.
3. The script prints per-tire verdict: OK / add / bleed, with the exact fill-to number (temperature-corrected, load-aware).
4. Filling warm at a station? check --hot — get the adjusted fill-to.
5. tread — record per-tire tread mm; get safety verdict and replace urgency (with wet-grip context, not just legal minimum).
6. schedule — monthly check reminder dates with next expected targets (accounts for season: expect −1..−2 psi entering winter).
7. Log every check: trends reveal slow leaks weeks before TPMS triggers.

Commands

text

python3 scripts/tire_coach.py placard --vehicle "Mazda3" --front 33 --rear 33
python3 scripts/tire_coach.py check --vehicle "Mazda3" \
  --lf 30.5 --rf 30.7 --lr 32.0 --rr 32.2 --temp-c 8
python3 scripts/tire_coach.py check --vehicle "Mazda3" --hot \
  --lf 34 --rf 34 --lr 35 --rr 35
python3 scripts/tire_coach.py tread --vehicle "Mazda3" --lf 5 --rf 4.9 --lr 3.2 --rr 3.1
python3 scripts/tire_coach.py schedule --vehicle "Mazda3"
python3 scripts/tire_coach.py log --vehicle "Mazda3"

Store: tire_coach.json next to the script (--store to override).

Pitfalls

1. Using the sidewall max as target — the #1 mistake. Max pressure is a tire capability limit; the car's placard is the engineered setting for contact patch, wear, and ABS/ESC calibration.
2. Checking after driving — even 1 mile of driving adds 1+ psi. Use the 3-hour rule, or the --hot correction.
3. Ignoring the spare — spacesaver spares run 60 psi and are checked approximately never.
4. Nitrogen folk-lore — dry air behaves nearly identically; the real benefit of nitrogen shops is that they set and check pressure. Monthly checks with plain air beat annual nitrogen.
5. Tread at legal minimum ≠ safe — 1.6 mm passes inspection but wet braking distance from 80 km/h can be ~40% longer than at 4 mm. Treat 4 mm as the practical replacement trigger, 3 mm as urgent for winters.
6. Equalizing all four to one number — many cars specify different F/R pressures; also a 2+ psi side-to-side gap on the SAME axle signals a leak or alignment issue, not a fill-everything job.

Example

Cold morning 8°C, placard 33 psi, measured LF 30.5: verdict "add to 33" with correction note — but afternoon fill-up at 22°C: fill-to becomes ~33.6 to land back at 33 when cold. Tread LR 3.2 mm in October: legal (>1.6) but flagged "wet-season risk — winter tires below 4 mm lose the compound advantage; replace before the wet season." Monthly log shows LR losing ~1 psi/2 weeks vs others' ~1/month: slow leak caught before roadside.