I know I'm necroposting, but I want to add my experience to the forum, and this seems a better place than starting a new thread.
I've been driving with musician's earplugs for nearly the entire 18 months that I have had severe tinnitus. I started out with custom Westone 16 dB, and moved to Earaser 19 dB SNR. Over the course of these 18 months, my tinnitus has worsened several times. I'm not sure if driving with earplugs had anything to do with it or not - should I have used higher rated earplugs or none at all? I drive a Hyundai Sonata that is relatively quiet, but probably not as quiet as a nice Lexus or BMW. It is certainly quieter than a compact or mid-size car, but does have a louder than average fuel pump since it is a direct injection engine (GDI).
One interesting thing that I have noticed after all this time driving with earplugs is that, when trying to drive without plugs - even at low speeds on side streets, the engine and road noise is excruciatingly loud to me now. I'm starting to wonder if I've caused myself some problems. I can handle the TV and many other every day sounds just fine, but the sounds while driving are now overwhelming without earplugs.
I do have some observations about the progression of my in-car sound sensitivity from the onset of my tinnitus.
Pre-severe tinnitus, I would listen to my car stereo (an Infinity 5.1) at 30-32 volume. This has nothing to do with dB levels but just a reference number basically. I haven't tested what dBA level that would be, but I'm guessing between 85-90 dBA. After my tinnitus became severe, I regulated the volume to 18-20 the first few days. Afterwards I wouldn't raise the volume higher than 16. After about 2 weeks I remember noticing the stereo felt too loud at 12, so I capped the volume at 10 max.
At this point I started contemplating wearing earplugs in the car as I started worrying the road noise and everything was worsening my condition. From then on, I've been wearing earplugs while I drive.
Currently, if I sit parked and turn on the stereo, it sounds too loud any higher than 6 now without earplugs. I basically can only handle 1/5 the volume now after 18 months.
I think I need to begin weening myself off, but I'm really unsure since my tinnitus hasn't been stable since it came about.