Tyres I: pressure, the placard and TPMS

A tyre on a US car can fall a quarter below the carmaker's pressure before the car's warning light is required to say so, and you probably won't see it. NHTSA, the US federal road safety regulator, says "Underinflated tires are visually difficult to detect", and it counted 511 people killed in tyre-related crashes in the US in 2024. It also says "only 19 percent of consumers properly inflate their tires".¹ Pressure is the check you'll make most often, and the usual ways it goes wrong are the wrong number, warm tyres, or no check at all because a light on the dashboard seems to have it covered. This lesson is about the right number, the right moment, what the light is and isn't required to do, and how to read the writing on the side of a tyre (the tire, in the US).

Two sections below state law: what the US requires of the warning light, and the law on pressure. This is general information, not legal advice. The rules differ between England and Wales, Scotland, Northern Ireland and each US state, and they change.

Two numbers, and whose each one is

Lesson 1, "What a car is made of, and why the handbook comes first", set out the two numbers: the carmaker's cold pressure on the placard or in the handbook, and the tyre maker's maximum moulded on the sidewall. NHTSA's rule of thumb from that lesson stands: "the correct pressure for your tire is what the vehicle manufacturer has listed, NOT what is listed on the tire itself."¹ This lesson starts from why the two numbers differ, because that's what lets you handle the cases the rule of thumb doesn't cover.

The sidewall figure answers a question about the tyre. The US federal standard for new car tyres, FMVSS 139, defines it as "the maximum cold inflation pressure to which a tire may be inflated".⁴ It's the same whatever car the tyre is fitted to.

The placard figure answers a question about the car. The US standard that requires the placard, FMVSS 110, ties the carmaker's pressure to the weight the car puts on each tyre. It says the car's maximum load on a tyre "shall not be greater than the applicable maximum load rating as marked on the sidewall", and that the car's normal load on a tyre "shall not be greater than 94 percent of the load rating at the vehicle manufacturer's recommended cold inflation pressure for that tire".³ On this course's reading, that means the carmaker chooses a pressure at which each tyre can carry what that car puts on it, with a margin. The same placard also carries the car's seating capacity and a line about weight: "The combined weight of occupants and cargo should never exceed XXX kilograms or XXX pounds", with the car's own figure in place of the X's.³

TyreSafe, a UK tyre safety charity backed by tyre makers, carmakers and retailers, lists what pressure affects: "the tyre's speed capability, load carrying capacity, handling response, wear rate and overall safety."¹⁰ Annex 6 of the Highway Code, the part of Great Britain's Code on looking after a vehicle, says "Your brakes and steering will be adversely affected by under-inflated or over-inflated tyres."⁷ Neither page explains how pressure produces those effects in a tyre, and this course read no source that does. What the federal standard says about the consequences is in the section on the warning light below.

Where the carmaker's figure is

In the US, FMVSS 110 requires the information "on a placard permanently affixed to the driver's side B-pillar", the upright post the driver's door closes against; for a car with no B-pillar it names other places nearby, starting with the edge of a door.³ NHTSA says the label gives the pressure in both kPa and psi.¹ AAA, the US motoring clubs that sell roadside assistance and run a repair network, adds that "On older cars the decal may be in the glove box or inside the fuel filler door."¹¹

In the UK, TyreSafe, the industry-backed charity, says to look "in your vehicle manual, on a plaque in one of the front door sills or the inside of your fuel cap."¹⁰ This course read no UK law that requires a placard, or says where it goes, so the handbook is the place that is always there.

One car can have more than one figure. Lesson 1 showed Toyota's handbook for the 2023 Corolla sold in the US listing its pressures by tyre type, four types in all, and not one pressure for them all. Annex 6's wording, "for the load being carried", matters for the same reason: where a handbook gives one figure for normal use and a higher one for a fully loaded car, the Great Britain rule points at the one for the load you're carrying. That last step is this course's reading of the rule's words; no handbook this course read was checked for a laden figure, so look in yours.

Cold, with a gauge

"Cold" has a definition. NHTSA says to check at least once a month, when the tyres are cold, "meaning that the car hasn't been driven for at least three hours."¹ Annex 6 gives the reason not to check straight after a drive: "Warm or hot tyres may give a misleading reading."⁷ AAA, whose clubs sell roadside assistance and run a repair network, puts a size on it: checking warm tyres "can result in a pressure reading of up to 5 psi higher than the recommended pressure."¹¹ Five psi is about a third of a bar (0.34 bar), on this course's conversion.

The weather moves the reading too. NHTSA's winter page: "As the outside temperature drops, so does tire inflation pressure."¹² Neither source gives a figure per degree, so this course gives none. On this course's reading, that means a tyre set right in September can read low by December without a leak, which is one reason to check through the year rather than once. Lesson 15, "Winter and summer", comes back to it.

How often

The sources agree on cold, and disagree on how often.

No source this course read gives a reason for weekly against monthly. Every one of them asks for at least a monthly check; weekly is the Highway Code's advice for Great Britain, and every two weeks the AA's.

The gauge

AAA, whose clubs sell roadside assistance and run a repair network, says the gauges "built into air hoses at gas stations are often abused and frequently inaccurate", and that digital and dial gauges "tend to be more accurate and easier to read, although a good pen/stick gauge will do the job as well."¹¹ NHTSA's advice is to keep one in the car: "A tire can suddenly lose pressure if you drive over a pothole or bump into a curb when you park."¹ Neither source ranks makes of gauge, and this course recommends none.

Taking a reading is the same with any gauge. AAA gives the steps: "Remove the tire's valve cap", then "Place the gauge over the tire's valve stem and press firmly so that no escaping air is heard", read it, and "Replace the valve cap."¹¹ The valve stem is the one the cap screws onto.

NHTSA's steps are short. Find the figure on the label or in the handbook. Check every tyre, spare included. If a tyre is high, release air "by gently pressing on the tire valve stem with the edge of your tire gauge". If it's low, work out the "missing" pounds and add them at a service station. Then check them all again.¹ For the adding, AAA says to "push the air hose into the valve firmly, until the air stops escaping" and to "Check the pressure every few seconds".¹¹ Checking and adding air is Yours. So is the spare, where it's carried inside the car. NHTSA and the federal manual text both name it, and it is easy to forget because it never touches the road.

That's the simple case, and the fix is to add 4 psi and write it down. The case that catches people is the next one.

A warm reading in Morpeth

A driver fills up at a forecourt in Morpeth after a 40-mile drive. While she's there she checks the tyres with her own gauge. The handbook's cold figure for her car is 32 psi (2.2 bar, again a figure for the example). The front tyres read 35 psi. Her first thought is that the tyres are over-inflated and she should let 3 psi out.

Work through it before you read on. What's the reading a reading of? What does each source say to do with it? And what would she find the next morning if she let the air out now?

The warning light: what the law requires it to do

This is general information, not legal advice.

In the US every new car has had a tyre pressure monitoring system for years. NHTSA: "All passenger cars, light trucks and vans that are model year 2008 or newer are required to come equipped with this feature."¹ The rule is FMVSS 138, which phased it in from 5 October 2005 and, with limited exceptions, applied it to every new vehicle it covers built from 1 September 2007.² NHTSA describes two ways a system can work: sensors in the tyres ("direct"), or the wheel speed and other sensors the car already has ("indirect").¹ Your handbook says which yours is.

What the standard requires is a trigger, and the trigger is set well below the placard pressure. The light must come on "not more than 20 minutes after the inflation pressure in one or more of the vehicle's tires, up to a total of four tires, is equal to or less than either the pressure 25 percent below the vehicle manufacturer's recommended cold inflation pressure, or the pressure specified in the 3rd column of Table 1 of this standard for the corresponding type of tire, whichever is higher".² Table 1 sets that floor by tyre type: 20 psi (140 kPa) for a standard-load passenger tyre ("P-metric" in the table), 23 psi for an extra-load one, and 29 or 35 psi for light-truck tyres of load range C, D or E. On standard-load tyres the 25 per cent point is the higher one whenever the placard figure is above about 26.7 psi (this course's arithmetic), and every example below assumes those tyres; on extra-load and light-truck tyres the floor can be the figure that counts.² The standard defines the system as one that warns when a tyre "is significantly under-inflated", and it does not say why a quarter was chosen as the line. This course read no source that does.

Read the chart row by row. On a car whose placard says 36 psi, the light isn't required until the tyre is down to 27 psi, and the whole gold stretch, 9 psi of it, is pressure you can lose without the standard requiring any warning. The band gets wider as the placard figure gets higher: 7.5 psi at 30, 10 psi at 40.

The standard also writes the lesson's point into your owner's manual. Every car certified to it must carry this text, among other lines: "Please note that the TPMS is not a substitute for proper tire maintenance, and it is the driver's responsibility to maintain correct tire pressure, even if under-inflation has not reached the level to trigger illumination of the TPMS low tire pressure telltale."² The same required text says what a low tyre does: "Driving on a significantly under-inflated tire causes the tire to overheat and can lead to tire failure. Under-inflation also reduces fuel efficiency and tire tread life, and may affect the vehicle's handling and stopping ability."² TyreSafe, the industry-backed charity, says the same about the light in fewer words: "TPMS is designed to be a critical warning system and not a reminder to check the vehicle's tyres."¹⁰

When it comes on, and when it flashes

NHTSA says that when the symbol comes on, "at least one of your tires is significantly underinflated", and to inspect the tyres and check the pressure "as soon as possible".¹ The federal manual text says "you should stop and check your tires as soon as possible".² Neither says how to decide where to stop; lesson 16, "When the car stops: where, who stands where, and the call", does that.

Two other patterns mean something different, and NHTSA explains both.

• On and off on a cold morning. "On cold mornings, the TPMS symbol may illuminate for a short period of time and then turn off." NHTSA's explanation: it's "likely caused by marginally low tire pressure that dips below the warning threshold overnight but rises to an acceptable level as the tires heat up".¹ So a light that goes out after a few miles has still told you something: on NHTSA's likely explanation, a tyre was below the system's threshold when cold. Check it cold with a gauge.
• Flashing, then steady. "A system malfunction may be indicated by a flashing of the TPMS symbol for 60 to 90 seconds with the warning lamp remaining illuminated after the flashing sequence." It repeats at every start until fixed, and NHTSA says: "You should contact your vehicle dealer for a system inspection."¹ The standard's manual text adds that while the malfunction light is on, "the system may not be able to detect or signal low tire pressure as intended", and that new tyres or wheels can be the cause.² Until it's fixed, the gauge is the only check you have.

Take a car in Pueblo whose TPMS symbol flashes for about a minute every morning and then stays on. The owner assumes a tyre is low, checks all four with a gauge, finds them at the placard figure, and decides the light is wrong. Half right: on NHTSA's account, that flashing pattern is how a system malfunction may show itself, not a low tyre. So the tyres are fine today, the system isn't watching them, and the fix is an inspection of the system (NHTSA says the dealer's). Having the system repaired is Call someone; checking the tyres by hand until then is Yours.

In the UK

This course read no UK law that requires a car to be fitted with a TPMS. A motoring organisation's date for UK fitment could not be checked against a regulation, so it isn't given. What was read is the MOT, Great Britain's periodic test. DVSA's inspection manual checks the system on "M1 vehicles first used on or after 1 January 2012", and M1 is the manual's category for passenger vehicles with "no more than 8 passenger seats in addition to the driver's seat", which takes in ordinary cars,¹⁷ and a system "malfunctioning or obviously not working" is a Major defect, which fails the test.⁹¹⁷ The manual warns testers that a lit lamp "does not automatically mean the system has a malfunction", since it stays on after a real pressure loss, and that "In the event of a system malfunction, the lamp may flash a number of times and then remain on", the same pattern NHTSA describes for US cars.⁹ Northern Ireland's test was not read on this point.

The law on pressure

This is general information, not legal advice.

In Great Britain

Annex 6 states it as a MUST rule, which the Highway Code's introduction says marks a legal requirement: "Tyres MUST be correctly inflated to the vehicle manufacturer's specification for the load being carried. Always refer to the vehicle's handbook or data."⁷¹⁴ The law Annex 6 cites for its tyre rules is regulation 27 of the Road Vehicles (Construction and Use) Regulations 1986, which says a car with pneumatic tyres "shall not be used on a road" if, among other things, "the tyre is not so inflated as to make it fit for the use to which the motor vehicle or trailer is being put".⁸ Notice that the regulation's own words don't name the maker's figure; the Highway Code's paraphrase does, and it tells you where the number comes from. Using a car whose tyres break a construction and use requirement is an offence under section 41A of the Road Traffic Act 1988, which covers "brakes, steering-gear or tyres".¹⁵ Lesson 20, "The law on a car's condition: Great Britain and Northern Ireland", covers the penalties.

So in Great Britain the law and the advice sit side by side. The Highway Code's statement of the law is that the tyres are at the maker's pressure for the load, every time the car is used; the regulation's own test is that they are inflated fit for the use. The advice, also Annex 6's, is to check weekly, cold. The law sets no interval; the interval is the advice.

At the MOT, DVSA's manual lists "Tyre obviously under-inflated" as a Minor defect, and "If a vehicle has only minor defects, it will pass its MOT inspection".⁹¹⁷ So a car can pass with a soft tyre, and the law on inflation still applies the next time it's driven. Northern Ireland has its own road traffic law, which this course did not read.

In the US

This course read no US federal or state law that sets the pressure a driver must run. The federal standards read here are rules for how cars are built and labelled: the placard (FMVSS 110) and the warning system (FMVSS 138). On this course's reading they bind the carmaker, and what they say to the driver comes as the owner's-manual text quoted above. One state's inspection shows the difference. New York's inspection reports pressure outside the maker's figure, and the DMV's page says: "This is an advisement only, and not cause for rejection."¹³ Your own state's rules are on its DMV or state police pages; lesson 22, "The US: state inspections, emissions tests and tread law", goes through five states.

Reading a sidewall

The side of a tyre carries more than its maximum pressure. Four things are worth being able to read.

The size. DVSA's manual uses an example to explain one part of it: "a 215/55R15 has an aspect ratio of 55%".⁹ The manual does not decode the rest of the string, and this course read no source that does, so it won't either. What you need is simpler: the whole size, copied exactly, matched against the sizes your handbook lists. The size matters because, as lesson 1 showed with Toyota's tyre types, the pressure you want can depend on it.

The maximum pressure. FMVSS 139 requires it on each sidewall, and it's the figure you don't inflate to.⁴

The date code. In the US the tyre identification number (TIN) follows the letters DOT. The rule is 49 CFR 574.5: "The date code, consisting of four numerical symbols, is the final group. The date code must identify the week and year of manufacture." The first two digits are the week and the last two the year, and the rule's own example is "0109 means the tire was manufactured in the first full calendar week of 2009".⁵ NHTSA adds a practical point: "Look on both sides of the tire. The TIN may not be on both sides."¹ DVSA's manual, reading the same kind of code on tyres in Great Britain, says "Tyres with a three-digit code will be more than 10 years old."⁹ What a tyre's age means for replacing it is lesson 8's question.

The quality grades (US). Tyres sold in the US for passenger cars carry three grades under the Uniform Tire Quality Grading standard, 49 CFR 575.104: TREADWEAR (a number), TRACTION ("AA, A, B, or C") and TEMPERATURE (A, B or C). The rule's own words limit what each means. Treadwear "is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test course", and "The relative performance of tires depends upon the actual conditions of their use, however, and may depart significantly from the norm due to variations in driving habits, service practices and differences in road characteristics and climate." Traction is a wet braking grade, and the rule warns it "is based on straight-ahead braking traction tests, and does not include acceleration, cornering, hydroplaning, or peak traction characteristics." The grades don't apply to deep-tread winter tyres or to space-saver spares.⁶ So a treadwear of 400 is not 40,000 miles or any number of miles. On the rule's own example (150 wears one and a half times as well as 100), it means that tyre would wear four times as well as a tyre graded 100 on the government's course, and nothing about your roads.

What uneven wear tells you

Pressure shows up on the tread, and the pattern points to the cause. AAA, whose clubs sell roadside assistance and run a repair network, gives three patterns:¹¹

• Worn in the centre: over-inflation. "Too much air pressure causes mostly the tire's middle section to contact the road."
• Worn on both edges: under-inflation. "Too little air pressure causes mostly the tire's outer edges to contact the road."
• Worn on one edge: alignment. "This typically occurs when the wheels are out of alignment."

TyreSafe, the industry-backed charity, says the same of over-inflation: it "will round out the middle of the tread pattern".¹⁰ Annex 6 widens the list of causes: "Excessive or uneven tyre wear may be caused by faults in the braking or suspension systems, or wheels which are out of alignment. Have these faults corrected as soon as possible."⁷ Reading the pattern is Yours. Having alignment, brakes or suspension looked at is Call someone, and lesson 6, "Brakes, steering and suspension", has the signs that go with them.

What people get wrong

"The light will tell me when a tyre needs air." In the US it's required only at 25 per cent under on standard-load tyres (at a higher floor on some others), within 20 minutes, and the law's own manual text says it "is not a substitute for proper tire maintenance".² This course read no UK rule that sets a trigger at all.

"I'd see a soft tyre." NHTSA: "Underinflated tires are visually difficult to detect."¹ A quarter low can look normal.

"The forecourt gauge is good enough." AAA, whose clubs sell roadside assistance and run a repair network, calls air-hose gauges "often abused and frequently inaccurate".¹¹ Your own gauge, used the same way every time, at least gives readings you can compare.

"The tyres were right in summer, so they're right now." "As the outside temperature drops, so does tire inflation pressure."¹²

"A warm tyre reading high means let some out." Warm tyres read high (up to 5 psi, says AAA, whose clubs sell roadside assistance and run a repair network), and the cold figure is the one that counts.¹¹

"Treadwear 400 means 40,000 miles." It's a comparison on a government test course, and the rule says real wear "may depart significantly from the norm".⁶

Practice

For your plan

Add the tyre page to your car file: the cold pressures (front, rear, spare, and any loaded figures) with where each came from; your trigger point; the tyre size; each tyre's date code, read as week and year; and a checks log with the date, the time the car had stood, each reading and what you did. Put the interval you've chosen at the top of the log, and whose advice it is.

Connections

Lesson 1 set out the two numbers, the placard and the sidewall; this lesson is how they're used. Lesson 8, "Tyres II: tread, damage, age, and when to replace", reads the rest of the tyre: the tread, damage and age, and the argument about when to replace. Lesson 12, "Fuel economy: what maintenance buys, and the figures that were withdrawn", comes back to what pressure does for fuel, and lesson 15, "Winter and summer", to what cold does to pressure. Lesson 18, "A flat tyre: the decision, and spare, space saver, sealant or run-flat", returns to the spare this lesson had you check.

Go deeper

• NHTSA, "Tires", free: the US regulator's pages on pressure, the TPMS symbols and what each pattern means, and the tyre grades. This course read it from an archived copy.
• FMVSS 138, free on the eCFR: read S4.2 and S4.5. The first is the trigger, and the second is the text your owner's manual is required to carry, which says what the light is not for.
• The Highway Code, Annex 6, free, for Great Britain: the tyre paragraph has a MUST rule and advice a few lines apart.
• AAA, "Tire Safety and Maintenance", free: the wear patterns and the gauge advice in one page. AAA's clubs sell roadside assistance and run a repair network.

Sources

1. NHTSA, "Tires". Read: full page, from Internet Archive capture 20260924154655 (nhtsa.gov refuses automated readers), re-read 26 September 2026; no later capture with different content exists. The live page is linked. US federal.
2. 49 CFR 571.138, FMVSS No. 138, "Tire pressure monitoring systems". Read: full section text, eCFR, re-read 26 September 2026 (version of 24 September 2026), including S4.2, S4.4, S4.5, S7 and Table 1. US federal law.
3. 49 CFR 571.110, FMVSS No. 110, "Tire selection and rims". Read: full section text; S4.2.1 and S4.3 re-read 26 September 2026 (version of 24 September 2026). US federal law.
4. 49 CFR 571.139, FMVSS No. 139, "New pneumatic radial tires for light vehicles". Read: S1 to S5.5; test procedures skimmed; S3 and S5.4 re-read 26 September 2026. US federal law.
5. 49 CFR 574.5, "Tire identification requirements". Read: full section, re-read 26 September 2026. US federal law.
6. 49 CFR 575.104, "Uniform tire quality grading standards". Read: (a) to (d) and the Figure 2 consumer text; test procedures skimmed; (c) and the Figure 2 text re-read 26 September 2026. US federal law.
7. The Highway Code, "Annex 6. Vehicle maintenance, safety and security". Read: full text, re-read 26 September 2026; updated 29 January 2022. England, Scotland and Wales.
8. The Road Vehicles (Construction and Use) Regulations 1986, regulation 27, as amended. Read: the whole regulation on the live page, 26 September 2026. England, Wales and Scotland.
9. DVSA, MOT inspection manual: cars and passenger vehicles, section 5, "Axles, wheels, tyres and suspension". Read: 5.2.1 to 5.2.3 in full, 26 September 2026; manual updated 1 June 2026. England, Scotland and Wales.
10. TyreSafe, "Check your pressures". Read: full text, re-read 26 September 2026; last updated 16 September 2026. A UK tyre safety charity backed by tyre makers, carmakers and retailers.
11. AAA Exchange, "Tire Safety and Maintenance". Read: full text, re-read 26 September 2026; undated. A US motoring club that sells roadside assistance and runs a repair network.
12. NHTSA, "Winter Weather Driving Tips". Read: full text, from Internet Archive capture 20260812080428; the tyre section re-read from the same capture 26 September 2026. US federal.
13. New York State DMV, "New York State Vehicle Safety/Emissions Inspection Program". Read: full page, from Internet Archive capture 20260710141515 (dmv.ny.gov refuses automated readers); the tyre items re-read 26 September 2026. New York.
14. The Highway Code, "Introduction". Read: full text; updated 1 July 2022. England, Scotland and Wales.
15. Road Traffic Act 1988, section 41A. Read: full revised text, re-read 26 September 2026. England, Wales and Scotland.
16. The AA, "10 essential car maintenance tips". Read: full text, re-read 26 September 2026; updated 10 July 2025. The AA sells breakdown cover and repairs. UK.
17. DVSA, MOT inspection manual: cars and passenger vehicles, Introduction. Read: the category definitions and the defect categories, 26 September 2026; manual updated 1 June 2026. England, Scotland and Wales.