[Porsche-O-Phile]

Three words: diesel school buses.

Specifically very old diesel school buses. No lie - most of the ones on our roads today are 1970s vintage. Even with the so-called "low-sulfur/clean diesel" blends, these things absolutely belch black soot like something out of a WW2 movie. I imagine there's a heck of a lot more total emissions from those and heavy trucking than motorcycles.

I actually wouldn't mind too terribly having to run a catylitic on the bike, but for now I just try to keep it tuned as best I can and when riding, keep it rolling and up to temp (minimize idling).