Under the EPA's 2024 tailpipe rule, a battery-electric car counts as zero in the fleet average for smog-forming exhaust, and the agency expected 26 percent of new light-duty vehicles in model year 2027 to be one. In a proposal published May 18, the agency cut that forecast to eight percent for 2027 and 12 percent for 2028.With most of those zeros gone, the EPA says gasoline cars can't be expected to make up the difference on the original schedule. It has proposed pushing the start of the tougher Tier 4 emissions standards for soot and smog-forming pollution from model year 2027 to 2029.The greenhouse-gas rules got the attention when the EPA rescinded them in February. The Tier 4 delay shows those carbon limits were also holding up a second set of rules, the ones that govern what comes out of a gasoline tailpipe besides CO2. Electric cars were how automakers planned to meet both. Take away the pressure to sell them, and the soot rule comes apart next, starting with the particulate filter that was supposed to start appearing under gas-powered cars in model year 2027.What the 2024 rule required, and what the EPA now proposesThe 2024 multi-pollutant rule had two pieces that matter to anyone buying a gas car. The first lowers the fleet-average limit for non-methane organic gases plus nitrogen oxides, the ingredients of ground-level ozone, from the Tier 3 level of 30 milligrams per mile in model year 2026 to 15 milligrams per mile by 2032. The second sets a particulate matter limit of 0.5 milligrams per mile that each vehicle has to meet on three lab tests, including a cold-start run at −7°C and the high-speed US06 highway cycle.For vehicles up to 6,000 pounds gross vehicle weight rating, the per-vehicle standards were to phase in at 20 percent of production in model year 2027 and 40 percent in 2028. The EPA's proposal replaces both years with 100 percent Tier 3, the rules in force today, and resumes the phase-in at 60 percent Tier 4 in 2029. Medium-duty vehicles get the same two-year delay. Comments closed July 6. As of this writing, the EPA's rulemaking page lists only the proposal, not a final rule.The agency calls this "Part 1." A separate Part 2, not yet proposed, will reconsider the Tier 4 program as a whole, and the EPA says that could include changes to the standards themselves, the lead time, the phase-in and the test procedures.How an electric car satisfied a soot rule for gas enginesThe fleet average is the easier mechanism to follow. Automakers certify each engine family to a "bin," and the sales-weighted average has to land at or under the limit. A car with no tailpipe pulls the average down the same way a low-mileage used car pulls down the average odometer reading on a dealer's lot. The proposal states the rule this way: "Each BEV counts as a zero for emissions in the NMOG+NOX fleet average standard." The EPA writes that it projected, and many manufacturers assumed, that the fleet average could be met primarily with electric-vehicle sales.The particulate standard is per-vehicle, but electric cars helped there too. Because they have no tailpipe PM, every one sold counted toward the percentage of production that had to meet Tier 4. The EPA's own example: with a 20 percent phase-in requirement for 2027 and a projected 26 percent electric share, a manufacturer might have relied heavily or even completely on electric vehicles to meet it. In that case, the number of gasoline cars that needed new hardware in the first year could have been zero.That hardware is a gasoline particulate filter, or GPF. It is a ceramic honeycomb similar to a catalytic converter substrate, except that alternating channels are plugged at opposite ends, so exhaust has to pass through the porous walls and leaves the soot behind. Diesels have carried the same kind of filter for years. On a gasoline engine, the exhaust usually runs hot enough to burn off the trapped soot during normal driving. The EPA's draft regulatory impact analysis puts the cost to the manufacturer at about $179 for a vehicle with the fleet's average 2.13-liter engine, in 2024 dollars, and between $137 and $372 across the range of engine sizes.A ceramic honeycomb substrate like this one is the base for both catalytic converters and gasoline particulate filters. In a filter, alternating channels are plugged so exhaust must pass through the walls. Photo: Oak Ridge National Laboratory, CC BY 2.0, via Wikimedia Commons.With the electric share at eight percent instead of 26, the arithmetic reverses. The proposal says manufacturers "would need to rely on unplanned modifications to large numbers of non-BEV vehicles" to comply, and it uses that to argue the Clean Air Act requires more time. Section 202(a)(2) says a new vehicle standard takes effect only after the period the EPA finds necessary to permit the development and application of the required technology, with consideration of the cost of compliance. The EPA's case rests on that clause, not on the rescission of the carbon rules. The market forecasts it cites trace to two events of 2025: Congress's disapproval of California's Advanced Clean Cars II waiver, signed in June, and the July 4 budget law that ended the federal electric-vehicle purchase credit early.The savings per car are smaller than the press release suggestsThe EPA's May 14 announcement promised $1.7 billion in savings and "hundreds of dollars saved per vehicle for American families." Administrator Lee Zeldin said, "Freedom is the foundation of this nation, and this includes the freedom to choose the car you drive."The preamble breaks the number down. The savings on smog-control hardware come to about $59 per vehicle, at a three percent discount rate, spread over 9.38 million vehicles in 2027 and 8.97 million in 2028. The savings from skipping particulate controls are about $171 per vehicle, but they apply to 1.22 million vehicles in 2027 and 2.85 million in 2028. By The Auto Wire's reading, a buyer reaches "hundreds of dollars" only on a car that would have needed both upgrades, where the two figures add up to about $230. Most of the 2027 cars in the EPA's analysis carry the $59 figure. The total is $1.77 billion at three percent and $1.66 billion at seven percent.Those 1.22 million and 2.85 million figures are roughly how many gasoline cars the EPA expected to need particulate controls in those two model years under the existing rule. Under the proposal, that requirement waits until 2029.The EPA also estimated what the delay costs in air quality. By its tables, it adds 389 tons of nitrogen oxides and 51 tons of fine particulate matter in calendar 2027, and 976 tons of NOx and 145 tons of PM2.5 in 2028. The NOx figures stay at or below 0.2 percent of on-road emissions. The particulate share climbs to 1.1 percent of on-road PM2.5 in 2035 and 1.6 percent in 2045, because cars built without filters in 2027 and 2028 stay in service for years after the rule itself catches up.Diesel pickups and big rigs are next in lineOwners who have had a diesel go into limp mode over diesel exhaust fluid will care more about the EPA's July 14 heavy-duty proposal. Selective catalytic reduction, the system that injects DEF into the exhaust to break down NOx, only works when the tank is full of the right fluid. Since the technology arrived, EPA rules and guidance have required "inducements": if the system detects an empty tank, bad fluid or tampering, the engine loses power and the truck is eventually limited to a crawl. The EPA now proposes to replace those derates with visible or audible warnings.The proposal covers newly built heavy-duty engines, nonroad diesel equipment, and light- and medium-duty diesel vehicles, which is where a heavy-duty pickup sits. The EPA says it is also considering new guidance for diesels already on the road, and it cites complaints from operators and from states including Iowa, Alaska and Nebraska. Comments closed August 29.For big-rig buyers, the same proposal would keep emission-related warranty periods at their model-year 2026 levels instead of lengthening them in 2027. It would push the longer regulatory useful life, equivalent to 650,000 miles for the heaviest engines compared with 435,000 miles now, back to model year 2030. And it would make nonconformance penalties available starting in 2027. Those let a manufacturer keep selling an engine that misses a standard as long as it pays a per-engine penalty. The EPA says some manufacturers' 2027 engine programs have run into technical problems. It is not reopening the 2027 NOx limits themselves.The EPA puts purchaser savings at up to $37 billion in warranty costs, including up to $6,000 per diesel vehicle, and says net savings could reach $12 billion once added operating costs are counted. A shorter emissions warranty lowers the sticker price and moves repair risk to the owner. When an SCR system or NOx sensor fails after the warranty ends, the fleet pays the shop, not the engine maker. With diesel at record prices, carriers will be doing that math carefully.The carbon case in court, and what it can't undoThe February rescission, which the EPA says will save "over $1.3 trillion" from 2027 through 2055 and more than $2,400 per vehicle according to its economic fact sheet, is before the D.C. Circuit. A coalition of states led by Massachusetts, California and New York, joined by cities and counties including Chicago, Denver, Los Angeles and New York City, petitioned for review on March 19. The Auto Wire has covered how a Supreme Court case complicates the agency's legal footing, and the separate fight over California's older waivers.A court win for the states would restore the greenhouse-gas standards on paper. It would not bring back the tax credit or California's 2035 sales rules, and those are the inputs behind the eight and 12 percent forecasts the Tier 4 delay relies on. The EPA has not proposed dropping the soot and smog standards, but their next version will be written for a fleet that is mostly gasoline.What Part 2 decides for 2029 and later carsFor someone shopping for a 2027 or 2028 gas car, the practical effect of Part 1 is small. If it is finalized as proposed, those cars will be certified to the same Tier 3 rules as a 2026 model, with no new filter requirement behind the price on the window sticker. The larger question is Part 2. The EPA has said it may change the Tier 4 levels and test procedures, and the 2024 rule requires the 0.5-milligram particulate limit to be met on the −7°C cold-start test and the US06 highway cycle as well as the standard lab cycle. How Part 2 handles those tests will decide whether a filter becomes standard equipment in 2029 or never arrives.The EPA's rule for gasoline cars was written on the assumption that a quarter of new cars would have no tailpipe. It now has to be rewritten for a market where about one in ten does, and the agency will have to decide whether the particulate filter is worth about $179 on every gas car or not worth requiring at all.Should the EPA require particulate filters on gasoline cars now that there are fewer EVs to help meet the soot standard, or is delaying the standard the right response to a market that changed?The post EPA Built Its Smog Rule Around EVs That Aren't Coming. Now It Wants Two More Years for Gas Cars appeared first on The Auto Wire.