I guess "large" is subjective. 1-5% is the ballpark I have in my head based on experience, which qualifies as "large" to me when I get passed by thousands of cars a day.

The hard numbers I'm aware of are about motorcycles, which have much higher rates of illegal modifications than other vehicles. This source documents a bunch of other sources, with estimates ranging from 40-70%:

https://noisefree.org/sources-of-noise/motorcycles/