California's tire rules: what actually passed

§1The short version

If you only want the answers to the questions everyone is asking, they are here. The rest of the page is the working.

§2Where the 70% comes from

Discount Tire — which trades as America's Tire in California and runs 151 stores there — told the Commission at the 10 June public hearing:

…if the Phase 2 requirements beginning in 2031 were applied to the tires we sell in California today, roughly 70 percent of currently available options would be removed from the market. John Baldwin, America's Tire / Discount Tire · TN270848 · transcript TN271437 p.68

Three things worth holding onto. It is one retailer's range, not the market. It counts available options in that retailer's range, which is Baldwin's own phrasing — not tires on the road. And it was measured against the April proposal, before the exemptions in §4 existed. The base 7.1 Phase 2 limit did not change; what changed is how many tires have to meet it.

Discount Tire is also the source of the Treadwell mileage data the Commission relied on to conclude that efficiency and tread life are unrelated. The same filing supports the rule and warns about its effect on choice.

§3The limits

Standards are a ceiling on the EU-correlated rolling resistance coefficient, in newtons per kilonewton. Lower is more efficient. A separate wet grip standard of 1.0 applies under §3308(a).

Where a tire qualifies under more than one category, §3306(g) applies the most lenient ceiling — and if it qualifies as an all-season winter performance tire, that exemption overrides everything else. Winter-type snow tires do not appear in this table at all, because they are not replacement tires — see §5.

For scale: the Commission puts the average rolling resistance of original-equipment tires at about 7.1, which is where the Phase 2 base number comes from. The statute requires replacement tires to be as efficient as OE on average, and the Resolution formally finds that the programme ensure[s] that replacement tires sold in the state are at least as energy efficient, on average, as tires sold in the state as original equipment on new passenger cars and light-duty trucks. The rule delivers that through a set of caps rather than an average, and Tire Reviews analysis I could not find a published sales-weighted calculation showing how the final mix of limits and exemptions produces that result.

§5The 3PMSF fork: two routes out

Both snowflake exemptions have the same first limb, pointing in opposite directions. The all-season winter performance tire must be subject to the UTQG standards; the winter-type snow tire must not be. UTQG is a passenger-car system and light-truck tires sit outside it, so the two definitions divide the 3PMSF market cleanly between them — and both sides come out of the standards.

The LT route is the more complete escape. A winter-type snow tire is struck from the definition of a replacement tire at §3302(a)(46)(D), so the Article simply does not reach it. The Initial Statement of Reasons identifies the statutory basis as PRC §25770(d)(2), which excludes winter-type snow tires from the definition of a replacement tire.

The 17 July amendment is what opened this

As proposed on 24 April, a winter-type snow tire had to satisfy three things: be limited in its acceptable use to winter periods by virtue of its design and construction; attain a snow traction index of 112 or more; and be studded or carry the Alpine Symbol. An all-terrain tire would have failed the first limb — it is not limited to winter use — and an otherwise-in-scope LT all-terrain would have stayed regulated at 9.0, then 7.8.

That limb was struck. The Commission's own summary of the change:

Struck subsection (a)(62)(A) to base the definition of “winter-type snow tire” only on the objective criteria of the originally proposed subsections (a)(62)(B) and (C). Notice of Additional Public Comment Period · TN271526

What remains is purely objective, and purely mechanical: not subject to UTQG, traction index of 112, the Alpine symbol on the sidewall. An LT-metric all-terrain carrying 3PMSF meets all three. USTMA had asked for exactly this — an objective, performance-based definition keyed to the FMVSS 139 traction index and the Alpine Symbol — and welcomed it in their final comments.

Why it matters more than the numbers suggest

The Commission's own test programme ranked All Terrain among the highest rolling resistance and the lowest wet grip of every category Smithers measured. That is the segment the snowflake now carries out of the rule in LT sizes, and out of the standards in passenger sizes.

It also removes the fallback that the efficiency advocates were relying on. ASAP, ACEEE, NRDC and UCS accepted the exemption on the understanding that Phase 1 reporting would generate the data needed to set a standard later. That works for the passenger-metric route, which still reports. It does not work for the LT route, which reports nothing at all.

And this is not really an all-terrain story. It is a 3PMSF story: the all-weather touring segment carries the snowflake too, and it is growing quickly. Any tire that earns the marking — in any size, by one route or the other — is outside the rolling-resistance and wet-grip standards.

One unresolved wrinkle

Both definitions turn on whether a tire is “subject to the UTQG standards”, and USTMA pointed out that this is harder to verify than it sounds:

…UTQG applicability is governed by federal law (49 CFR 575.104) and may not always be readily verifiable by regulators from the tire sidewall alone. Furthermore, no LT/C tires are subject to UTQG requirements. U.S. Tire Manufacturers Association · TN271795 §III.D, p.7 of 15

They recommended keying it to whether the tire actually carries UTQG markings. The Commission adopted the text unchanged, so which of the two routes a given tire takes rests on a federal scope question that cannot always be answered by looking at the tire.

One argument that does not work: the rule borrows the UTQG treadwear test procedure for light-truck tires at §3303(f), but borrowing a procedure does not change which tires are federally subject to the UTQG system. Pirelli objected on exactly that basis — The UTQG federal regulation does not pertain to LT tires so this prescription should be deleted and the info in the database should not be required.

USTMA read the final structure the same way, and went further. Noting that only 16 light-truck tires were tested across both Smithers phases, they added that some were understood to be 3PMSF-marked, which would exclude them from the applicability of the proposed regulation and further narrow the sample size of the Smithers data.

§6The 1.45 trap

Ultra-high-performance tires get the most generous offset in the rule — 8.5 instead of 7.1 in Phase 2. But the concession is gated on an attribute that one popular sub-segment is built to trade away.

§3302(a)(62) defines ultra-high-performance as a tire that carries a W, Y or (Y) speed symbol — (W) was deleted on 17 July — is capable of 168 mph or more, and has a relative wet grip braking performance index of at least 1.45. Miss 1.45 and the tire is not UHP under the rule at all. It falls to the base standard — 7.1, the strictest number in the regulation — if no other category or exemption applies.

Tire Reviews analysis Many summer UHP tires should be better positioned to clear 1.45. Non-3PMSF ultra-high-performance all-season is the class most exposed: it trades wet grip index away in exchange for cold-weather capability, but without the snowflake it never reaches the exemptions in §5, and it is generally not optimized for rolling resistance either — which is why I think it is the segment most exposed here. Pirelli put the objection in the record:

…the required wet grip braking performance index for ultra high performance tires is too demanding. Not all UHP tires are designed to have superior wet performance but rather some UHP tires focus on dry performance. Pirelli Tire LLC · TN271788, p.3 of 5

Unchanged in the final rule. Commission staff described the design intent at the June hearing: we structured the regulations such that increasing wet grip provides a compliance benefit, whereas decreasing wet grip will make compliance more difficult for tires with high speed ratings. The effect is deliberate. Whether the segment can absorb it by 2033 is the open question.

Track tires are fine

§3302(a)(11) excludes competition tires from the definition of a replacement tire. A tire must be designed for motorsport use and intended for competition fitment, and meet all four of: tread depth 8/32″ or less; speed rating W, Y or (Y); UTQG treadwear not greater than 200; and not classified as an all-season. That last criterion is what stops a 200-treadwear all-season claiming the exclusion.

§7The tread-life question

This is the part that decides whether the rule is good for drivers, and it is the part with the thinnest evidence behind it. Public Resources Code §25773(a)(1)(C) requires the Commission to show the standards do not adversely affect average tire life.

What the manufacturers said

USTMA recognizes the real and quantifiable performance tradeoffs that generally exist among rolling resistance, tread life, wet traction, and cost. A regulation requiring a lower rolling resistance will generally result in tires with decreased tread life as well as potentially reduced wet traction. U.S. Tire Manufacturers Association · TN270674 §VIII, p.15 of 16

What the Commission concluded

The Staff Report finds …no meaningful relationship between treadwear and efficiency… (p.55), resting on a scatter of rolling resistance against UTQG treadwear with an R² of 0.0194 (Figure 8, p.53) and against Discount Tire's Treadwell mileage estimates at R² of 0.0045.

Three reasons to hold that loosely

No controlled wear testing was done. Smithers measured rolling resistance and wet grip. Treadwear entered the record as Uniform Tire Quality Grading (UTQG) Treadwear Rating as identified by tire stamping on sidewall. The two wear datasets are manufacturer-declared sidewall grades and a proprietary retailer database.

CEC finding The Staff Report itself acknowledges that the sidewall grade can understate the underlying test result. Explaining why the long-life thresholds sat above every sidewall rating in the dataset, the Staff Report says manufacturers are able to underreport the results of the test procedure for public disclosure and tire labeling and that the actual laboratory results of longer wearing tires exceed the sidewall values (p.28). Tire Reviews analysis Using a noisy, downward-biased proxy can attenuate an observed correlation — which is a reason to hold the pooled result loosely, not proof that a relationship was hidden.

Several disaggregated subsets showed materially stronger relationships than the pooled analysis. Smithers reported these by tire category and the Staff Report did not carry them forward:

Smithers gives the direction for three of these groups: rolling resistance directionally increased with increasing UTQG Treadwear Rating… The R² of the Economy tires was 0.528… the correlation… for the Fuel Efficient tires was stronger, as indicated by a higher R² value of approximately 0.660. The Efficient usage group is described the same way. For the remaining categories Smithers reports the correlations as weak and does not state a consistent direction.

Continental made the statistical point directly:

A low R² does not demonstrate the absence of a physical relationship… The relevant question is not population-level correlation but design-level constraint. Continental Tire the Americas · TN270669

Where the offsets came from

The Commission is explicit that the long-life allowances are a hedge, not an admission: they were added as an additional precaution and to reward longer-wearing tires. Their existence therefore does not by itself prove a trade-off exists.

Tire Reviews analysis The narrower criticism still stands: the Staff Report does not derive the specific +0.4 / +0.7 / +0.8 / +1.4 N/kN allowances from any measured engineering slope. Their stated basis is feedback from manufacturers and state policy.

Two further wrinkles. Qualification turns on the raw UTQG wear test score, which is not the number on the sidewall; the database records whether a tire clears the threshold rather than the score itself, so consumers will not necessarily see the underlying figure — though the declaration is made under penalty of perjury and the Commission retains verification authority under §3309. And §3306(b) puts ultra long-life in the same bucket as ultra-high-performance and run-flat tires, neither of which has any treadwear floor at all.

Finally, tread depth — the most direct lever on both rolling resistance and tire life — must be reported under §3305(e)(8), but the rule sets no general minimum new-tread depth. The wet-grip test is a new-tire procedure. So neither requirement directly measures causal tread life or worn-state grip. Smithers found tread depth against rolling resistance at R² = 0.742 for fuel-efficient tires, the strongest rolling-resistance relationship in its Phase 1 Table 5A.

§8Does the maths work for drivers?

California's headline is a net benefit of $153–$207 per vehicle over the four-year life of a set of tires.

Fuel savings are fixed by distance driven — 10,413 miles a year, about 41,652 miles over four years. If tread life falls by a fraction f, a driver buys 1/(1−f) sets to cover the same distance. Break-even is f = net ÷ (set price + net):

For a $600–$1,000 car or SUV set, roughly 13–20% shorter tread life would erase the Commission's $153 estimated net benefit. For the truck and van case the equivalent range is roughly 17–26%.

Tire Reviews analysis The Commission is not short of tread-life evidence: it has UTQG correlations, Treadwell mileage estimates and the published literature. What the record does not contain is a direct estimate of the causal tread-life change, if any, that follows when a manufacturer redesigns a currently non-compliant tire to reach Phase 2 while trying to hold wear, wet grip, cost and the rest of the performance envelope. Continental made exactly that cross-sectional-versus-causal distinction in the docket. That is the argument — not that the rule will cost drivers money, but that the analysis assembled does not answer the question that decides it.

§10What happens next

Phase 1 applies to tires manufactured on or after 1 January 2029; Phase 2 from 1 January 2033. The Commission is required to review the programme at least once every three years under PRC §25773(d), in consultation with CalRecycle.

Six things I would want to see at the review

1. Controlled wear testing of a matched set spanning the Phase 2 rolling-resistance range — the one thing this rulemaking never produced.
2. A minimum tread depth, or a wet-grip test at worn depth. As adopted, the grip floor is a new-tire test and the RTEP sets no general minimum new-tread depth.
3. A treadwear floor on the UHP and run-flat tier, or separation of those categories from ultra long-life.
4. Publication of the UTQG wear test score used to claim a long-life offset, so the public can independently verify the claim — the Commission already retains verification authority of its own.
5. Verification against real-world wear at the triennial review, with the Phase 1 checkpoint USTMA asked for as the trigger for revisiting Phase 2.
6. A re-run SRIA on the adopted 2029 / 2033 schedule, with tire life modelled rather than assumed.

The final rule is markedly more sensible than the original proposal — the phase-in is realistic, the tiering recognizes that different categories face different constraints, and a wet grip floor is a genuine safety backstop. Whether manufacturers can hit these numbers without giving up tread life, performance, price or choice is the thing to watch, and the answer will arrive as market data long before it arrives as regulation.