In short. No law defines a semi-slick: its approval mark decides. In the UK, a car tyre may only be sold with an E mark under UN Regulation 30, and the MOT accepts a tyre marked "Not for Highway Use" only if it carries that mark. In the US the mark is the DOT symbol, and Pennsylvania's inspection, for one, rejects any tyre marked "for racing purposes only". In New South Wales, tyres must meet ADR 23, which accepts UN Regulation 30 as equivalent. A road-legal semi-slick then follows the rules of any tyre: 1.6 mm in the UK, 1.5 mm in Australia, 2/32 in in most US states. The makers say these tyres are made for a dry track, fear standing water and need to warm up, and they start with much less tread than a road tyre.
What "semi-slick" means, and where the law draws its line
"Semi-slick" is a sellers' and drivers' word for a track tyre with few grooves and a soft compound. Makers use it too: Yokohama files its ADVAN A052 under "Semi-Slick with racing performance" and calls it "Street Legal, Track Ready".
The texts draw a different line: between a road tyre and a competition tyre. UN Regulation 30, which approves passenger car tyres, covers new tyres for cars and light trailers, but "does not apply to tyres designed for (a) the equipment for vintage cars (b) competitions" (paragraph 1, as reproduced in ADR 23/02, Appendix A). The UK supply rules exempt "a tyre constructed solely for use on vehicles in competitions and bearing words or letters which indicate that use", moulded in at manufacture (Motor Vehicle Tyres (Safety) Regulations 1994, regulation 8). And in the US, the DOT symbol "must not appear on tires to which no Federal Motor Vehicle Safety Standard is applicable" (49 CFR 574.5).
So a "semi-slick" is either a road tyre carrying an approval mark, with a track compound, or a grooved competition tyre with no road approval. Only the sidewall tells you which.
How to read the sidewall
| Marking on the sidewall | What it shows | Where it counts |
|---|---|---|
| A circle around the letter E and a number, then an approval number | Tyre type granted approval under UN Regulation 30 (paragraphs 5.4.1 and 5.4.2) | UK, Europe; accepted as equivalent by ADR 23/02 in Australia |
| DOT followed by the tyre identification number | The maker's statement that the tyre conforms to the applicable US federal safety standards (FMVSS 139, S5.5; 49 CFR 574.5) | US; Canada has its own standard, TSD 139, and its own national safety mark |
| UTQG grades (TREADWEAR, TRACTION, TEMPERATURE) | US quality grading under 49 CFR 575.104; not an approval | US information only |
| "Not for highway use", "NHS", "For racing purposes only", "For competition use only" | Made for competition | In the UK, acceptable at the MOT only with an E mark; in Pennsylvania, a reason for rejection |
| Size, load index, speed symbol, four-digit date | The usual markings | Everywhere; see how to read a tyre |
Table by the Onyx Forged design office, from the texts cited in each row.
Regulation 30 requires the approval mark to be "clearly legible and be indelible" (paragraph 5.5). If you cannot find it, ask the seller before you buy.
United Kingdom
Sale. Regulation 4 of the 1994 tyre safety regulations says "no person shall supply" a new tyre that can be fitted to a passenger car "unless the tyre is marked with an approval mark" under ECE Regulation 30 or the equivalent EC directive. Competition tyres marked as such are exempt.
The 2014 date that forums mention. Regulation (EC) 661/2009, kept in force in Great Britain, says that from 1 November 2014 the Secretary of State shall "prohibit the sale and entry into service" of new tyres that do not meet its Annex II (Article 13(5)). Annex II, Part A sets a wet grip index of at least 1.1 for a normal passenger car tyre (Annex II). A 2015 trade article reported that many track day tyres were "effectively outlawed for road use" after that date (TyreTrade.ie); the text behind the date is Article 13.
The MOT. The inspection manual (section 5.2.3, page updated 14 October 2026) says testers "should only accept tyres with NHS, Not for Highway Use or similar markings if they have an 'E' marking and a number contained within a circle". A tread depth outside the rules is a dangerous defect; tyres of different sizes or different structures on one axle are major defects. The wider list of wheel and tyre checks is in our guide to new wheels and the MOT.
On the road. The Construction and Use Regulations 1986 require at least 1.6 mm "throughout a continuous band comprising the central three-quarters of the breadth of tread and round the entire outer circumference" (regulation 27(4)(f)). The same regulation forbids a tyre that "is unsuitable having regard to the use to which the motor vehicle … is being put", and one not inflated to be "fit for the use". Regulation 26 forbids tyres "of different types of structure" on the same axle. None names semi-slicks; they apply as to any tyre.
United States
Federal level. FMVSS 139 "applies to new pneumatic radial tires for use on motor vehicles" of 10,000 lb GVWR or less (49 CFR 571.139, S2), and each tyre must carry the DOT symbol, which S5.5 treats as the maker's statement that the tyre conforms to the applicable federal safety standards. FMVSS 139 has no wet braking test of the kind Europe uses.
UTQG. Passenger car tyres also carry quality grades under 49 CFR 575.104. The traction grade is measured in wet braking on a test surface: AA needs an adjusted traction coefficient above 0.54 on asphalt and 0.38 on concrete; C is 0.38 or less on asphalt, or 0.26 or less on concrete. Limited production tyres (no more than 15,000 a year of one design and size, among other criteria) are exempt, so some track tyres carry no grades.
State level. Tread depth and roadworthiness are set by each state. A 2006 survey in Traffic Injury Prevention found that "most states require a minimum of 2/32 of an inch", two require less and some have none (TRID). Pennsylvania's inspection rules are an example of how a state treats a competition tyre: the inspector rejects a tyre that "is marked 'not for highway use,' 'for racing purposes only' or 'unsafe for highway use,' or has a similar designation", or with "two adjacent treads with less than 2/32-inch tread remaining at any point" (67 Pa. Code § 175.80). We have not read every state's code; check yours.
Australia, and New South Wales
NSW. Transport for NSW writes that "tyres fitted to your vehicle must meet the construction standards set out in Australian Design Rules (ADR) 23", and that vehicles registered in NSW must also meet the dimensional standards of the Tyre and Rim Association of Australia (VSI.09, Rev. 5, 28 February 2019). It adds: "Never mix tyre types or sizes on an axle". A change of more than 7 per cent in overall tyre diameter needs a compliance check under VSI.06.
ADR 23 and the E mark. ADR 23/02 states that UN Regulation 30 "shall be deemed to be equivalent" to its technical requirements (clause 7). An E-marked tyre therefore meets the construction standard. The ADR text we read names no US standard as an alternative; we do not know how a NSW inspector treats a tyre that carries only a DOT symbol, and the tyre maker or a licensed vehicle engineer is the one to ask.
The national model rules. The Australian Light Vehicle Standards Rules, a national model law that the states use as the basis for their own rules, require "at least 1.5 millimetres deep in all principal grooves" across the tyre width that normally touches the road and round the whole circumference (rule 52), forbid different carcass constructions on one axle (rule 48), and require a car's tyres to be rated for at least 180 km/h or the car's top speed if lower (rule 50).
Victoria says it in one sentence: every tyre on a road wheel must be of a type built, and vouched for by its maker, for normal road use (VSI 16, October 2021).
Canada
New radial tyres for vehicles of 4,536 kg or less must conform to Technical Standards Document 139, the Canadian counterpart of FMVSS 139 (Motor Vehicle Tire Safety Regulations, section 5, current to 21 September 2026); section 7 of the same regulations sets a Canadian national safety mark. Tread depth is provincial; we could not read the Ontario text and give no figure.
The rules side by side
| United Kingdom | United States | Australia (NSW) | Canada | |
|---|---|---|---|---|
| Mark to look for | E mark under UN R30 (supply rule, 1994) | DOT symbol (FMVSS 139) | ADR 23 construction; UN R30 accepted as equivalent | Federal tyre standard (TSD 139); national safety mark |
| Tyre marked for competition | MOT accepts it only with an E mark | No DOT symbol where no FMVSS applies; Pennsylvania rejects it | Outside UN R30; Victoria: a type built for normal road use | Provincial; not read |
| Minimum tread | 1.6 mm, central three-quarters, whole circumference | Set by each state; 2/32 in for most | 1.5 mm in all principal grooves | Provincial; not read |
| Same axle | Same structure; MOT: same size | Pennsylvania: same size and construction | Same type and size (VSI.09); same carcass (rule 48) | Provincial; not read |
Table by the Onyx Forged design office from the texts above, read on 16 October 2026.
Rain and cold: what the tyre makers say
Michelin calls its Pilot Sport Cup 2 R "a DOT legal track tire optimized for DRY track use" and warns that "on wet tracks or roads, track tires, especially in a worn state, can be highly impacted by hydroplaning"; it asks drivers to slow down on wet roads with the driver aids switched on (Michelin). Pirelli says its P Zero Trofeo R is "intended mainly for track use on dry surfaces" and that, since it is a road-legal tyre, the driver must follow local traffic rules (Pirelli).
Two labels measure wet braking, both on a new tyre. In Britain and Europe a normal tyre needs a wet grip index of 1.1 to be sold, and the label grades the result: Yokohama lists the ADVAN A052 in class A or B in most sizes, C in one. In the US, the UTQG traction grade gives a coarser view. Neither says anything about standing water or a worn tyre, which is where Michelin's warning applies.
On cold, none of the maker pages we read gives a temperature figure. Michelin writes that the Cup 2 R "requires a warm-up phase to reach optimal performance"; Pirelli puts the warm-up at about 20 km on track, depending on the car and the circuit. We do not know of a published lower limit for these models, so we do not give one. A semi-slick is not a winter tyre, and in places where winter tyres are required, it does not meet that rule; see our guide to winter tyres in the UK.
Less tread to wear: our reading
Michelin publishes the new tread depth of every size in 32nds of an inch. We converted it to millimetres (× 25.4 / 32) and took off the legal minimum.
| Tyre (Michelin US data) | New tread | In mm | Usable above 1.6 mm (UK) | Above 1.5 mm (Australia) |
|---|---|---|---|---|
| Pilot Sport 4 S (road sport tyre), most sizes | 9.5/32 | 7.5 | 5.9 mm | 6.0 mm |
| Pilot Sport Cup 2, most sizes | 7.2/32 | 5.7 | 4.1 mm | 4.2 mm |
| Pilot Sport Cup 2, the other sizes | 5.5 to 6/32 | 4.4 to 4.8 | 2.8 to 3.2 mm | 2.9 to 3.3 mm |
| Pilot Sport Cup 2 R, nearly all sizes | 5.5/32 | 4.4 | 2.8 mm | 2.9 mm |
Reading and calculation by the Onyx Forged design office, product pages read on 16 October 2026: 32 of 40 Cup 2 articles at 7.2/32 (the other 8 between 5.5 and 6/32), 42 of 44 Cup 2 R articles at 5.5/32 (2 at 5/32), 138 of 224 Pilot Sport 4 S articles at 9.5/32. Above the US 2/32 in threshold (1.59 mm), the figures are the UK ones, except the Pilot Sport 4 S at 6.0 mm.
A Cup 2 at 7.2/32 has 69 per cent of the usable tread of a Pilot Sport 4 S at 9.5/32; a Cup 2 R, 47 per cent. Wear rate also depends on compound and use. A track day can take a thin tyre below the limit, which is why Pirelli asks you to check that the remaining depth meets the "legal minimum for use on public roads (1.6mm / 2/32")" before driving home.
Pressures: one setting for the road, another for the track
On the road, nothing changes: the pressure is the car maker's, set cold. Michelin and Pirelli both say so for their track tyres. Our guide to tyre pressure explains where to find the figure.
On track, the makers describe the same method with different numbers:
| Source | Start | Warm-up | Hot setting | Before the road |
|---|---|---|---|---|
| Michelin, Pilot Sport Cup 2 R | Car maker's road pressures | "a steady run of laps" | 2.3 to 2.7 bar (33 to 39 psi) hot, depending on car and track, sometimes more; not below 1.9 bar (28 psi) cold | Car maker's pressures |
| Pirelli, P Zero Trofeo R | Car maker's recommended set-up | About 20 km | Bring back to the target if exceeded, at each stop | Tyres cooled, car maker's pressures, at least 1.6 mm (2/32 in) |
Michelin adds that sidewall markings wearing away mean the pressure is probably too low (add at least 0.2 bar, 3 psi), and that negative camber on track should not exceed 4 degrees.
Why the pressure climbs: our calculation
At a roughly constant volume, the absolute pressure of the air in a tyre (gauge reading plus 14.7 psi) is proportional to its absolute temperature (°C plus 273.15). A tyre set to 30 psi at 20 °C reads:
| Air temperature inside the tyre | Gauge reading |
|---|---|
| 5 °C (cold morning) | 27.7 psi |
| 20 °C (set in the garage) | 30.0 psi |
| 40 °C | 33.0 psi |
| 60 °C | 36.1 psi |
| 80 °C | 39.1 psi |
| 100 °C | 42.2 psi |
Calculation by the Onyx Forged design office. The air temperatures on track are assumptions: none of the sources we read publishes them. In bar, 2.0 bar at 20 °C becomes 2.41 bar at 60 °C and 2.82 bar at 100 °C.
That is about 1.5 psi (0.1 bar) per 10 °C. A reading taken after a session cannot be compared with the door pillar figure, and bleeding a hot tyre down to the cold road figure leaves it under-inflated once it cools.
The wheel: width, fixings, heat
A semi-slick does not need a special rim width, only the one its size calls for. On the 21 sizes that the Pilot Sport Cup 2 and the Pilot Sport 4 S share, Michelin publishes the same rim width range for both: for example 9 to 10.5 in for 265/35 ZR19. The width follows the size, not the tread pattern. A size change also moves the overall diameter, which the tyre size calculator compares, and in NSW more than 7 per cent needs a compliance check. Onyx Forged does not recommend a size or an offset: the reference figures are those of your car's maker, and how to choose aftermarket wheels explains what each figure does.
On track the wheel heats up with the brakes. Check the nuts or bolts at the car maker's torque with the car on the ground, as described in our guide to wheel nut torque. If you work under the car, it must be raised and resting on axle stands. A bulge, a cut or a chunk missing from the tread after a session is a reason to have the tyre checked before you drive on the road again; our guide to sidewall bulges explains why. The full list of checks before and after a session is in our track day checklist.
Frequently asked questions
Are semi-slick tyres road legal in the UK?
Yes, if they carry the E mark under UN Regulation 30, and then they follow the same rules as any tyre: 1.6 mm in the central three-quarters, the same structure on an axle, suitable for the use. A competition tyre without an E mark escapes the supply rule only as a competition tyre, and the MOT does not accept it if it is marked "Not for Highway Use".
Are semi-slick tyres road legal in NSW?
Yes, if they meet ADR 23: Transport for NSW requires it, and ADR 23 accepts an E mark under UN Regulation 30 as equivalent. Then 1.5 mm in all principal grooves and no mixing of types or sizes on an axle. A tyre designed for competition falls outside UN Regulation 30.
Are semi-slick tires street legal in the US?
If they carry the DOT symbol, the maker states that they meet the federal standard. Tread depth and inspections are up to each state; Pennsylvania rejects any tyre marked "for racing purposes only" or "not for highway use", whatever else is on the sidewall.
Can I drive semi-slicks in the rain?
They are legal in the rain, but Michelin and Pirelli describe them as dry-track tyres, and Michelin warns about hydroplaning, more so when worn. Michelin asks drivers to slow down on wet roads with the driver aids switched on. The wet grip class on the label lets you compare two models.
What hot pressure should I run on track?
The one in the tyre maker's data and the car's handbook. For the Cup 2 R, Michelin gives 2.3 to 2.7 bar (33 to 39 psi) hot, never below 1.9 bar (28 psi) cold. Before driving home, let the tyres cool and go back to the car maker's road pressures.
How long do semi-slicks last?
None of the maker pages we read gives a mileage. A Cup 2 starts with about 69 per cent of the usable tread of the same maker's road sport tyre, a Cup 2 R with about 47 per cent.
Can I keep semi-slicks on in winter?
No maker page we read gives a minimum temperature, and they are not winter tyres. Where winter tyres are compulsory, they do not count as such.
If you are planning a set of forged wheels, designed to JWL/VIA standards, to the figures of your car: customise a set.
Sources
- UK: Motor Vehicle Tyres (Safety) Regulations 1994, regulations 4 and 8; Regulation (EC) 661/2009, Article 13 and Annex II, as they apply in Great Britain; Road Vehicles (Construction and Use) Regulations 1986, regulation 26 and regulation 27; MOT inspection manual, section 5.2.3 (updated 14 October 2026).
- US: 49 CFR 571.139 (FMVSS 139), 49 CFR 574.5 and 49 CFR 575.104 (UTQG), via the Legal Information Institute; 67 Pa. Code § 175.80; Legal Minimum Tread Depth for Passenger Car Tires in the U.S.A.: A Survey (2006, abstract).
- Australia: ADR 23/02, Passenger Car Tyres, including UN Regulation 30 in Appendix A; Australian Light Vehicle Standards Rules 2015 (as at 22 November 2019), rules 48, 50 and 52; Transport for NSW, VSI.09, alternative wheels and tyres (Rev. 5, 28 February 2019, copy hosted by a car club); Victoria, VSI 16, tyre maintenance and repair (October 2021).
- Canada: Motor Vehicle Tire Safety Regulations, SOR/2013-198, section 5.
- Tyre makers: Michelin US, Pilot Sport Cup 2, Pilot Sport Cup 2 R and Pilot Sport 4 S; Pirelli UK, P Zero Trofeo R; Yokohama Europe, ADVAN A052.
- Trade press: TyreTrade.ie (31 July 2015)
All pages were read on 16 October 2026.
The Onyx Forged design office.