In the wake of the Volkswagen diesel emissions scandal, the U.S. Environmental Protection Agency (EPA) will expand on-road emissions testing to all diesel vehicles.
The agency currently conducts only a handful of on-road tests of new vehicles each year, to confirm laboratory results.
But Volkswagen's cheating and recent fuel-economy over-estimations by Ford, Hyundai, and Kia have led to criticism about the laboratory testing regimen.
The need to find out if other manufacturers have installed "defeat device" software that allows cars to cheat laboratory tests has galvanized the EPA to expand on-road testing, reports The New York Times.
The first tests included diesels from other Volkswagen Group brands, and found that about 10,000 VW, Audi, and Porsche vehicles using a 3.0-liter diesel V-6 also had illegal software.
Volkswagen has denied the EPA's assertion that these vehicles are non-compliant.
2014 Audi A7 TDI
The agency will not disclose the specific testing conditions to carmakers, which in theory should make cheating harder.
Regulators still reportedly believe that road tests are still too imprecise to completely replace laboratory testing.
Instead, the aim is to validate laboratory results, which are recorded by strapping a car to a dynamometer and running it under highly-controlled conditions.
Lab tests are conducted by the carmakers in compliance with EPA standards.
2015 Volkswagen Golf TDI
European regulators also plan to add on-road testing, although for somewhat different purposes than their U.S. counterparts.
While the EPA is primarily looking to expose cheaters, the European Union wants to close the widening gap between its tests results and real-world fuel economy.
However, when on-road testing becomes mandatory in 2017, carmakers will be allowed to emit more than twice the current European limits for nitrogen oxides, with stricter requirements coming into effect in 2021.