If you go to Kuala Lumpur, it is noticeable not a single vehicle - bus, truck or car - blows any smoke at all.
The problem in Thailand is enforcement of emissions from vehicles is a joke. Slip 200 baht to the tester to turn a blind eye to the smoke-belching vehicle, problem solved.
I get my elderly Vios registered in Phan. While it conforms, thanks to a good mechanic, I've lost count of the sh!(heaps that pass inspection at the testing station, when it is obvious they should not even be on the road.