In short. "All-terrain" (AT) and "mud-terrain" (MT) are sales names that no regulation defines; what decides how a tyre behaves and which rules apply is printed on its sidewall. An all-terrain tyre is a road tyre with a more open tread: in Europe it usually carries the tyre label, and many carry the three-peak mountain snowflake. Many mud-terrain tyres are marked POR, "professional off-road", a UN category that requires at least 11 mm of tread, a void-to-fill ratio of at least 35 % and a speed rating no higher than Q (160 km/h, 100 mph). POR tyres sit outside the European label and the noise limits, and an MT marked only M+S does not count as a winter tyre where the law asks for the mountain snowflake, as Quebec does. One maker's label data put its all-terrain tyre 2 to 3 dB louder outside the car and at least 18 % higher in rolling resistance than its road SUV tyre: roughly 2 to 4 % in fuel on our arithmetic. Many AT and MT sizes for heavy 4x4s are LT (light truck) tyres, which carry their load index only at high pressure. Put side by side, an AT with the snowflake gives up less on tarmac and meets more winter rules; a POR mud-terrain trades noise, fuel, top speed and winter legality for grip in deep mud and on rock.
What separates an all-terrain from a mud-terrain tyre
UN Regulation No. 117 (revision 4) sorts tyres by use; the quotations are our translation of the French text published by the TNPF.
- A special-use tyre (2.12) is designed "both for road and off-road use". A passenger-car (C1) or light-commercial (C2) tyre needs a tread with "larger and more widely spaced block elements" than a normal tyre, a tread depth of at least 11 mm and a void-to-fill ratio of at least 35 % (6.6).
- A professional off-road tyre (2.13) is a special-use tyre "mainly designed for use in severe off-road conditions". It must meet the same 11 mm and 35 % and have a speed rating of Q or lower (6.7), and it is marked "POR" (4.2.7).
- A tyre for severe snow conditions (2.11.1) has passed the snow test and may carry the three-peak mountain snowflake.
We read the data one maker publishes size by size for its best-known pair, in the UK and the US, on 16 October 2026:
| All-Terrain T/A KO3 (AT) | Mud-Terrain T/A KM3 (MT) | |
|---|---|---|
| New tread depth (US sizes) | 15/32 or 16/32 in (11.9 or 12.7 mm) | 18/32 in (14.3 mm) |
| Speed rating (UK sizes) | S on 52 sizes, R on 6, Q on 2 | Q on all 37 road sizes |
| Category | Road tyre, label class C2 (one size, 31x10.50R15 109Q, is POR) | POR, explained by the maker with the three criteria of 6.7 |
| EU tyre label | Published for 60 of 61 sizes (not the POR one) | None |
| Snow marking | Mountain snowflake and M+S on 60 sizes | M+S only |
| US mileage warranty | 50,000 miles | None listed (6-year limited warranty) |
That is one maker's pair. Some MTs are not POR, and some ATs lack the snowflake: read the sidewall and the data sheet of the exact size.
Construction. Both ranges are mostly LT sizes. BFGoodrich's US construction page calls ply rating "a legacy term for load capacity": the load index and the load range letter (C, D or E, required on an LT sidewall by FMVSS 139) are the way to compare strength.
What the POR mark takes a tyre out of
The tyre label. Regulation (EU) 2020/740, applied since 1 May 2021 and kept in Great Britain's law after Brexit, names "off-road professional tyres" first among the tyres it does not apply to (Article 2(2)(a)), and defines them as "a special-use tyre used primarily in severe off-road conditions" (Article 3(4)). That is why the KM3's pages show no label class and no decibels.
Noise and rolling-resistance limits. UN Regulation 117 excludes tyres for professional off-road use from its scope (1.1.8), and Regulation (EC) No 661/2009, in the version kept in UK law, says its rolling-resistance and noise requirements "shall not apply to off-road professional tyres" (Article 9(7)).
Speed. A Q rating means 160 km/h, or 100 mph in Continental's table. What follows from that depends on where you drive:
- Great Britain. The MOT manual's load and speed rating checks are headed "Classes 5 and 7 only"; for a car (class 4) the section we read sets no speed-rating check. The Construction and Use Regulations still forbid a tyre that "is unsuitable having regard to the use to which the motor vehicle or trailer is being put or to the types of tyres fitted to its other wheels" (regulation 27(1)(a)). Different sizes or structures on one axle are major MOT defects (5.2.3 (b) and (c)).
- United States. FMVSS 139 covers radial tyres for vehicles up to 10,000 lb but excludes "light truck tires with a tread depth of 18/32 inch or greater"; those fall under FMVSS 119. A new KM3 at 18/32 in sits exactly on that line. The uniform tyre quality grades (UTQG: treadwear, traction, temperature) apply to "new pneumatic tires for use on passenger cars" and not to "deep tread, winter-type snow tires" (49 CFR 575.104(c)), so on our reading LT tyres carry no UTQG grades to compare.
- Australia. Rule 50 of the Australian Light Vehicle Standards Rules (the model law, as at 22 November 2019) asks for tyres rated for at least 140 km/h on "a car with special features for off-road use", with a four-wheel drive given as the example, or the vehicle's top speed if lower. A Q-rated tyre meets that figure. Rule 48 bans different carcass constructions on one axle. Each state applies the rules through its own law, which we did not read.
On the road: noise, wet grip and fuel
Where a label exists, read it with one caution: every all-terrain size we read is class C2, the light-commercial class, and 661/2009 defines C2 as a tyre with a single load index of 121 or less and a speed symbol of N or higher (Article 8). The letters on a C2 label do not use the C1 scale (Annex I to 2020/740):
| Class | Rolling resistance, C1 (N/kN) | Rolling resistance, C2 (N/kN) | Wet grip index, C1 | Wet grip index, C2 |
|---|---|---|---|---|
| A | 6.5 or less | 5.5 or less | 1.55 or more | 1.40 or more |
| B | 6.6 to 7.7 | 5.6 to 6.7 | 1.40 to 1.54 | 1.25 to 1.39 |
| C | 7.8 to 9.0 | 6.8 to 8.0 | 1.25 to 1.39 | 1.10 to 1.24 |
| D | 9.1 to 10.5 | 8.1 to 9.0 | 1.10 to 1.24 | 0.95 to 1.09 |
| E | 10.6 or more | 9.1 or more | 1.09 or less | 0.94 or less |
The annex adds that a tyre in both classes is graded on the scale of the higher one (C2, not C1). Compare letters only between tyres of the same class.
Noise is different again. The label gives decibels and a class set against the tyre's limit value (LV): A if the noise is at most LV − 3, B up to LV, C above it (Annex I, Part C). The limits come from Annex II of 661/2009: for a C1 tyre wider than 245 and up to 275 mm, 72 dB(A), plus 1 dB for snow or extra-load tyres; for C2, 72 dB(A) for a normal tyre and 73 for a traction tyre, plus 2 dB for special use and 1 or 2 dB for snow. A size of the KO3 rated B at 75 dB therefore has a limit of at least 75 dB, which the annex allows a C2 tyre only once its snow or special-use allowances are added. Compare the decibels, not the letters.
Our reading of the three ranges on BFGoodrich's UK site, 237 sizes in all:
| Range | Sizes | Class | Rolling resistance | Wet grip | Noise outside | Snow |
|---|---|---|---|---|---|---|
| All-Terrain T/A KO3 | 60 labelled of 61 | C2 | E (all) | C (all) | 74 dB (22) or 75 dB (38), class B | Snowflake and M+S |
| Mud-Terrain T/A KM3 | 47 | POR | Not published | Not published | Not published | M+S only |
| Advantage 2 SUV (road) | 129 | C1 | B (126) or C (3) | A (all) | 70 to 73 dB, class B | Neither |
In one size both ranges share, the KO3's LT265/65R17 116/113S shows E, C and 74 dB; the road tyre's 265/65R17 112H shows B, A and 72 dB.
What the figures mean, with our arithmetic shown:
- Noise. Decibels are logarithmic: a gap of x dB is an energy ratio of 10^(x/10). Two decibels more (74 against 72) is 1.58 times the sound energy; three (75 against 72) is 2.0 times. This is a pass-by measurement outside the car, not the noise in the cabin, and none of the sources we read measures cabin noise.
- Wet grip. C on the C2 scale is an index of 1.10 to 1.24, A on the C1 scale 1.55 or more; the scales differ, so we do not turn that into a stopping distance.
- Fuel ("mpg"). A class E in C2 means at least 9.1 N/kN; a class B in C1, at most 7.7. The smallest possible gap is 9.1 / 7.7 = 1.18, so at least 18 % more rolling resistance; class E is open-ended, so the upper bound is unknown. The US Transportation Research Board found that a 10 % cut in average rolling resistance "promises a 1 to 2 percent increase in the fuel economy" of passenger vehicles. Scaling that linearly, which is our assumption, 18 % more resistance costs roughly 1.8 to 3.6 %: a vehicle doing 20 mpg would fall to about 19.3 to 19.6 mpg. Heavier tyres and a larger diameter are not in that figure, and for an unlabelled MT no number can be given. On noise, 661/2009 itself says (recital 23) that the block pattern and damage-resistant materials of special-use tyres make them noisier.
These are one maker's ranges on one day; another brand may show different values.
In snow: M+S is not the mountain snowflake
M+S is a manufacturer's statement. The three-peak mountain snowflake requires a test: in the US, FMVSS 139 defines a snow tire as one that reaches a traction index of at least 112 against the reference tyre in the ASTM F1805-20 test and carries the Alpine symbol; Canada's Motor Vehicle Tire Safety Regulations define a winter tire in similar terms, by a snow traction index and the symbol (section 5(4)). An AT with the snowflake and an MT with M+S only are therefore treated differently wherever the law names a marking:
| Where | What the rule asks | AT with snowflake | MT with M+S only |
|---|---|---|---|
| Great Britain | No winter-tyre requirement in the texts we read; see our guide to winter tyres in the UK | No rule to meet | No rule to meet |
| Quebec | Winter tyres from 1 December to 15 March on all motor vehicles registered in Quebec; a tyre must show the pictogram or be studded; fine of $200 to $300 (Québec.ca, updated 5 October 2026) | Complies | Does not comply |
| British Columbia | On designated highways, M+S or the snowflake, with at least 3.5 mm of tread; at least two matching winter tyres on the drive axle, even on a 4x4 (province's page, last updated 12 November 2021) | Complies | Complies by marking |
| Rest of Canada | Transport Canada advises the snowflake symbol, sets of four, and not using tyres worn close to 4 mm on snow | Meets the advice | Does not meet it |
| Colorado | Traction Law, enforced on I-70 from Morrison to Dotsero from 1 September to 31 May, and on any state highway when CDOT puts it in force: 3/16 in of tread and a tyre marked with the snowflake, M+S or M/S, or chains or an alternative traction device on CDOT's list; $100 fine plus a $33 surcharge, $500 plus $157 if a lane is closed | Complies | Complies by marking |
| Australia | No winter-tyre requirement in the national model rules we read | No rule to meet | No rule to meet |
British Columbia's page is five years old; check the province's current rules before winter. Dates, studs and designated routes are detailed in our guides to winter tyre laws in Quebec and British Columbia and to chain and traction laws in California and Colorado. For the Alps and the rest of Europe, our guide to snow chains and snow socks in Europe covers the rules and the room chains need around a large tyre.
Load: LT tyres on a Defender or a G-Class
Many AT and MT sizes for heavy 4x4s are LT sizes with a double load index: 123/120 reads 123 for single fitment and 120 for twin fitment. BFGoodrich's sidewall guide says that LT tyres have stiffer construction and higher load capacity, and that fitting a P-metric tyre where LT is specified "leaves you short on load capacity". The trap runs the other way too: an LT tyre carries its nominal load only at its reference pressure, which is much higher than a car tyre's.
Continental's Technical Databook gives, for each LT size, the load per axle at each pressure (ETRTO values). For an LT 265/75 R16 123/120, single fitment, we divided by two to get the load per tyre:
| Pressure (bar) | 2.5 | 3.0 | 3.5 | 4.0 | 4.5 | 5.0 | 5.5 |
|---|---|---|---|---|---|---|---|
| Pressure (psi, our conversion) | 36 | 44 | 51 | 58 | 65 | 73 | 80 |
| Load per axle (kg) | 1,780 | 2,020 | 2,240 | 2,480 | 2,720 | 2,880 | 3,100 |
| Load per tyre (kg) | 890 | 1,010 | 1,120 | 1,240 | 1,360 | 1,440 | 1,550 |
| Share of nominal load | 57 % | 65 % | 72 % | 80 % | 88 % | 93 % | 100 % |
Index 123 is 1,550 kg in the same book's load index table. At 2.5 bar this tyre carries 890 kg, about what an index 103 (875 kg) or 104 (900 kg) tyre carries. The same page lists 7J to 8J as the permitted rim widths for this size, which matters if you change wheels.
For a Defender or a G-Class, the handbook and tyre placard give the sizes, load index and pressures the car maker lists; an LT tyre's pressure for the axle load comes from the tyre maker's table, confirmed by a tyre professional. This guide gives no figure for either car: the numbers that count are in your own handbook and on your own placard. Australia's rule 46 requires tyres able to carry the axle's share of gross mass at no more than the tyre maker's recommended pressure; the MOT checks load rating only on classes 5 and 7. Our guide to reading a tyre size, load and speed index gives the full index table, and tyre pressure explains where the car maker's figures are printed.
Choosing without asking the wrong question
Onyx Forged does not recommend a size or an offset, nor a tyre. The order of checks that follows from the texts above:
- The car's documents: the sizes, load index and speed rating listed for your car.
- The sidewall: POR, snowflake, M+S, LT, load range. They decide which rules apply.
- The label, comparing decibels and class (C1 or C2), not only letters.
- Your real use: share of tarmac, cruising speed, snow-law routes.
- Room under the arch: a taller or wider tyre can rub (wheels or tyres rubbing); our tyre size calculator shows the change in diameter.
- The pressure: the car maker's for the original fitment, the tyre maker's table for an LT, confirmed by a professional.
For any check under the car, raise it and set it on axle stands, never on the jack alone. If a tyre has been fitted at a pressure that does not match its load, have it checked before you drive on.
Frequently asked questions
What is the difference between all-terrain and mud-terrain tyres?
They are sales names. An AT is a compromise between road and track; in our example it has a European label and the mountain snowflake. An MT has deeper tread and larger voids, and many are POR under UN Regulation 117: at least 11 mm of tread, 35 % voids and a speed rating of Q or lower.
Are mud-terrain tyres louder on the road?
POR tyres publish no noise figure, so the label cannot say. The all-terrain range we read is 74 or 75 dB outside the car against 70 to 73 dB for the same maker's road tyre, and 661/2009 says block-pattern special-use tyres are noisier.
Which uses more fuel, all-terrain or mud-terrain?
For the AT we read, rolling resistance is at least 18 % above the road tyre's, roughly 2 to 4 % more fuel on our estimate. For an unlabelled MT, no figure exists.
Are all-terrain or mud-terrain tyres better in snow?
The rules care about the marking. A tyre with the mountain snowflake has passed a snow traction test; M+S alone has not. In Quebec only the pictogram (or studs) counts; British Columbia and Colorado also accept M+S.
Can I put mud-terrain tyres on a Defender or a G-Class?
Only if the size, load index and speed rating match what the car maker lists, at the pressure the load needs. Check the handbook and placard; in Australia, also the 140 km/h minimum.
Why does my mud-terrain tyre have no EU label?
Regulation 2020/740 does not apply to off-road professional tyres (Article 2), and the same tyres are exempt from the noise and rolling-resistance limits.
What do LT, load range and 123/120 mean?
LT is a light-truck size; the load range letter (C, D, E) is required on the sidewall in the US; 123/120 is the load index for single and twin fitment, reached at the tyre's reference pressure: 1,550 kg at 5.5 bar but 890 kg at 2.5 bar for an LT 265/75 R16 123/120.
Can I mix all-terrain and mud-terrain tyres?
Not on the same axle if they differ in size or construction: the MOT lists both as major defects (structure there means radial against cross-ply), and Australia bans different carcass construction on one axle. Transport Canada advises against mixing tread patterns, and British Columbia asks for two matching winter tyres on the drive axle.
Wheels for a 4x4 that carries its load
Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards. The design office works from the axle loads in your car's documents, and the tyres, their pressures and their fitting stay with your tyre fitter. To see how a set is put together, read our guide to choosing aftermarket wheels or go straight to customise a set.
Sources
- Regulation (EU) 2020/740 on tyre labelling, as kept in UK law: Articles 2, 3 and 20, Annex I (legislation.gov.uk).
- Regulation (EC) No 661/2009, as kept in UK law: Articles 8 and 9, Annex II Part C.
- UN Regulation No. 117, revision 4, French text published by the TNPF (our translation).
- GOV.UK, MOT inspection manual, section 5.2.3; Road Vehicles (Construction and Use) Regulations 1986, regulation 27.
- BFGoodrich UK: All-Terrain T/A KO3, Mud-Terrain T/A KM3, Advantage 2 SUV. BFGoodrich USA: KO3, KM3, tyre construction, reading a sidewall.
- Continental, Technical Databook Car, 4x4, Van 2015-2016, pages 8, 62 and 63.
- eCFR: 49 CFR 575.104 (UTQG), 571.139 (FMVSS 139), 571.119 (FMVSS 119).
- Canada: Motor Vehicle Tire Safety Regulations, section 5; Québec.ca, winter tyre obligations (in French); Transport Canada, using winter tires; British Columbia, tire and chain requirements.
- Colorado Department of Transportation, traction and chain laws.
- Australian Light Vehicle Standards Rules 2015, model law as at 22 November 2019.
- Transportation Research Board, Special Report 286, Tires and Passenger Vehicle Fuel Economy (2006).
All pages were read on 16 October 2026.
The Onyx Forged design office.