The 40–60% Rule: How to Choose a Tyre Speed Rating

The research behind the rule

This rule came out of a research programme, not a hunch. Selling tyres honestly meant I needed to know what a speed rating actually certifies, so that is where the work started: how the test behind the letters is run, what failure modes it screens for, what a tyre has to survive to earn one letter over the next, and why manufacturers build certain constructions to certain letters in the first place. That last question turned out to matter most, because letters cluster by construction type. A low profile carcass earns W or Y almost as a by-product of how stiff it has to be, while a light truck carcass earns Q or R because load capacity and puncture resistance take priority over speed.

From there the work widened. I went through braking statistics and performance data across construction types, set against speed index, trip length and time at speed. I studied wear in our own inspection programme: recorded wear conditions and defects on tyres from short trip, small town drivers against long distance drivers, and compared them against size, vehicle weight, UTQG temperature grade and the other variables we log on every tyre that crosses the hoist. On the technical side I collected manufacturer specification data, modern and historical: European and Japanese data books, a good number of them translated line by line, alongside print data books from the 1970s with specification tables from manufacturers like Bridgestone and Dunlop, which show how constructions and their letters have shifted over fifty years.

And I researched the law itself. Not just what New Zealand requires, but where the requirement came from and whether it still fits. NZ incorporates the European tyre regulations by reference, and the speed symbol system is a European convention written around European conditions, including roads with no speed limit at all. Applied to a country capped at 100 and 110 km/h, that inheritance leaves a wide gap between what the letters certify and what any driver here can use, and nobody had published a rule for reasoning inside that gap.

Across all of it one pattern held: at New Zealand speeds, on ordinary vehicles, the letter is set by how the tyre is built rather than by anything the driver will ask of it, and capability beyond roughly double your cruising speed goes unused by design. I wrote that observation down as a rule, then spent a long time trying to break it: against the inspection manual, against every vehicle type that comes through the workshop, and against every tyre in our catalogue. The cases it does not cover are named in its scope. For the general cases, passenger cars, sedans, wagons, SUVs, utes and 4WDs on standard tyres, it holds.

The rule in full

First, the ladder, because none of this makes sense without it. Speed letters do not run alphabetically. The road letters climb like this, in km/h:

H sits between T and V. It kept its old position from the days when H simply meant "high speed", and U (200) barely exists in the real market. So when this rule says H give or take a letter, it means one step on this ladder: T below, V above. Never G, never J.

Now the pocket version:

And the full version, with its edges stated out loud:

• Who it is for: ordinary cars, sedans, wagons, SUVs, utes and 4WDs on standard passenger tyres. It does not apply to light truck, all terrain, mud terrain, C-marked van, motorhome or winter tyres. Those letters are set by carcass and purpose, and a Q-rated mud tyre on a ute is the right tyre wearing the right letter. Space savers are excluded too: the 80 km/h sticker is the limit there, not the letter.
• Your anchor: the fastest speed you regularly sustain, and never less than 100, because the market does not build tyres for slower anchors anyway. Expressway commuters anchor at 110. Towing? Your anchor is 90, the legal towing limit.
• The band: your anchor should sit between 40% and 60% of the tyre's rated speed. At 100 km/h that endorses letters from R (170) to V (240), with H in the middle. At 110, W (270) earns its way into the band.
• The 60 line, the margin side: above it, step up to the next letter actually sold in your size. On passenger tyres this line almost never fires, which is fine. It never orders you above your placard: if the placard itself sits above the line, the manufacturer already made that call.
• The 40 line, the money side: below it, nothing is unsafe. You are paying for capability you cannot legally use, so pay nothing extra for it. If your size only comes in W or Y, and most low profile sizes do because the letter follows the stiff carcass rather than the compound, then the lowest letter offered is the band for your size. What to check instead of the letter: that the tyre is a touring or comfort pattern rather than a track compound, if daily driving is the job.
• Loaded or towing: keep the letter, check the load. Fit at least your placard's load index, XL where it was fitted originally. A faster letter adds zero carrying capacity at any legal NZ speed. The load rating guide covers this properly.
• The placard wins by default. The band's job is to tell you what you can sensibly drop to when the placard letter carries a price premium, not to argue with your manufacturer.

Worked examples, on real NZ numbers

• Corolla on H at 100 km/h: 100 ÷ 210 = 48%. Right in the middle of the band. This is why H is the default answer for most of the country.
• Swift on T at 100: 100 ÷ 190 = 53%. Ideal. Nobody needs to upsell this driver.
• Expressway commuter at 110 on W: 110 ÷ 270 = 41%, inside the band. The rule moves with how you actually drive.
• Ranger on Q-rated mud terrains: out of scope. The letter came with the carcass, and it is legal on every road in New Zealand. Yes, legal. Next section.
• Anything on Y at 100: 33%. Not unsafe, just not worth a premium. If it is the cheapest tyre in your size, buy it happily.

What the law actually requires (less than you have been told)

Nearly every guide in the industry says the placard letter is your legal minimum. It is not. The NZTA inspection manual's speed rating rule says a tyre fails a WOF only if its rating is

The measure is the road, not the placard. Every road letter from J (100 km/h) up passes in New Zealand, and the Q-rated ute fleet of this country is legal because the rule as written passes it, not because inspectors look away. The full legal breakdown, including what the same-axle rule does and does not cover, is in the speed rating guide.

That gap between law and placard is where this rule lives. The law would wave through a J. The placard might demand a W. The 40-60% rule names the sensible floor in between, T or H for most cars, which is the one thing neither the law nor the placard will tell you.

The letter is not a price tag, and our own stock proves it

The old story says high letters mean expensive performance rubber. Our live stock says the opposite. As I wrote this, of the 307 tyres in stock and ready to fit in Te Puke:

• W is our most common letter, 83 tyres, and our cheapest median at $180 fitted and balanced.
• T, squarely inside the band, has a median of $240 fitted.
• In more than four out of five cases where the same size is offered in two letters, the higher letter is the cheaper tyre.

The reason is construction. Most low profile sizes can only be built on a stiff carcass, and a carcass that stiff earns a W or Y certification whether the tyre is a track tyre or a budget commuter. The letter follows the construction, and the price follows the market. That is the whole argument of the rule in one line: the letter is a certification, not a grade, and not a price.

What this rule is, and what it is not

Before publishing I tested the rule against the inspection manual, against every edge case I could construct, and mechanically against every tyre in our own catalogue. Its limits are part of the rule, stated here on purpose:

• The band is soft. Nothing real separates 59% from 62%. It is "roughly", permanently. Anyone drawing bright lines between adjacent letters has missed the point.
• The percentages are not physics. A speed rating is a pass or fail certification, and 48% of one is not an engineering quantity. The band is a way of pointing at the right shelf, and it should never dress up as a thermal calculation.
• The 60 line rarely fires on car tyres. Its real work in New Zealand is done by the 40 line, the money line. A rule that mostly saves people money has nothing to apologise for.
• It is a buying rule. Not law, the law is in section 04, and not a safety threshold. It decides what the letter is worth paying for, nothing more.

Has anyone published a rule like this before?

I looked hard, because a claim of "I created this" needs checking before it gets published. There are percentage rules in the same territory. European type approval derives a tyre's required rating from a car's top speed with a fixed formula. The American trailer tyre standard runs a banded speed schedule with a hard ceiling. The European standards manual tapers load capacity above reference speeds. All of them anchor on the vehicle's capability or on the tyre's limit.

None of them anchor on the driver. As far as I can find, no tyre maker, retailer or motoring body anywhere has published a rule that starts from your actual cruising speed and hands back a band of sensible letters. If someone shows us prior art, we will link it here with credit. That is a standing offer. Until then: created at a tyre counter in Te Puke, from years of research and the same conversation over and over.

Sources

New Zealand law and where it comes from

• NZTA Vehicle Inspection Requirements Manual, in-service WOF and CoF, section 10-1, tyres and wheels: the speed category rule, the marking rule and the same-axle requirements, checked against the manual in August 2026.
• Land Transport Rule: Tyres and Wheels 2001, and NZTA’s list of standards incorporated by reference, which is how the European, American and Japanese tyre standards apply here.
• The 1958 UNECE Agreement (Geneva), which New Zealand acceded to in 2001. This is the lineage of our speed rating rules: a European system, written for European roads, adopted here by reference.

Testing standards and technical data

• UN ECE Regulation 30 (passenger) and Regulation 54 (commercial): the load and speed endurance procedures every letter is earned under.
• ETRTO, the European Tyre and Rim Technical Organisation: speed symbols, reference speeds, and the load capacity taper above 210 km/h, as republished in manufacturer data books.
• UTQG grading (US NHTSA): treadwear, traction and temperature grades, used throughout our wear comparisons.
• Manufacturer specification data, modern and historical: current European and Japanese data books, many translated for this research, and print data books from the 1970s carrying specification tables from manufacturers including Bridgestone and Dunlop.
• Published independent testing on braking and temperature behaviour across compound and construction types, for example Tyre Reviews’ controlled testing from 0°C to 15°C.

First-party data

• Our tread inspection programme: 1,500+ documented inspections with recorded wear conditions, defects, tread profiles, DOT dates and UTQG figures, compared against size, vehicle weight and usage patterns. Method and findings on the research hub.
• Live stock data: 307 fitted-available tyres in our catalogue at time of writing, letter counts and median fitted prices, August 2026.
• Every letter photograph on the speed rating guide comes from that same inspection archive. Photographs © Taylor Houghton.