No, there are different standards, between road vehicles, mobile power trains, fixed, emergency, etc. Emergency and fixed tend to have the most liberal allowances. Public road vehicles the most stringent. Tractors are somewhere in-between.

Of course the government exempts itself from this because they know all that bullshit of "it's just as reliable etc" is trickery. It's common to buy previously government owned deleted vehicles, because the people working in federal government don't want to eat the dogfood the plebs do, and AFAIK if you buy it from them then you're allowed to have it.

What kind of government vehicles are you referring to that are typically modified to legally avoid emission standards? Short of military or emergency vehicles, I’m not aware of This as a practice.

The ones I've seen, IIRC, were from DHS (CBP, etc) or various border operations. They pop up on auction every so often, but I don't have the links off hand.

I know people used to look for retired police cruisers back in the day because they thought they were “detuned” but I’m pretty sure that was a resale marketing myth.

The hardware package is slightly different in PI vs CV along with in some cases a tune. The HP would vary from anywhere from 10 to 20 from Civilian to Police.

Generally though, you'd also want a police vehicle because of how well maintained fleet vehicles are in general, along with some added features and viewing the idle engine hours.

My then-12yo neighbor had a hankering to drive his minibike on the road... which was illegal. He then discovered this loophole, and carried a basket of eggs from his mother's chickens. When stopped, he told the cops this was a "farm vehicle", because he was delivering them to neighbors atop the ridge.

They laughed and let him go. Technically, he was legal.

Realistically, he was learning lies work. (He's now a perpetually homeless drug abuser with an increasing criminal record - before somebody jumps to cheer for his cleverness.)