Ideally, it shouldn’t matter which vehicle is considered. In practice, it can be different, especially for single-track (bicycles etc.) vs. two-track (cars etc.) vehicles. This is a weakness of the smoothness tag, and has been discussed further up in this thread and here
You’re overinterpreting “usable” again. Of course you don’t need high clearance, but without high clearance this road cannot be used “normally” either (so you have to reduce speed significantly). So IMHO this is a case of very_bad sett.
(Reminder: “Usable by” is interpreted as “comfortably usable” rather than “just passable”.)
In my opinion the best way would be uploading it to Wikimedia Commons at https://commons.wikimedia.org/
They will have some annoying questions, but it is actually benefit - especially part about stating license and authorship. You can also upload to OSM Wiki - but it requires having account there with few edits and existing for few days.
If you are really unwilling to do either there are workarounds but it would be much cleaner for you to upload own photos.
(main annoyance is caused by copyright applying by default - and sadly “I’m ok with them being used freely” is not clear enough, you need to apply one of licenses to make them really reusable.
Options range from Template:CC-BY-SA-4.0-self - OpenStreetMap Wiki and Template:CC-BY-4.0-self - OpenStreetMap Wiki to Template:CC0-self - OpenStreetMap Wiki (just please do not use NC variants of CC licenses).
Thanks, I have to remind myself of it all the time too 
I think even with high clearance, this road cannot be used “normally” and you have to reduce speed significantly. The problem you would experience here with any car, high clearance or not, is that the suspension would be damaged if you drive too fast. I don’t think the suspensions of high clearance cars are significantly stronger than those of normal cars. That’s why I think example pictures for smoothness=very_bad should show obstacles that are so high that there is an advantage of having high clearance (reduced risk of the obstacle hitting the bottom of your car).
Thanks! However I think grass pavers are a form of pavement, so should be in the “paved” table.
Will look for worse smoothness grass paver surfaces.
main surface page for years listed them as paved, until Template:Map Features:surface: Difference between revisions - OpenStreetMap Wiki
I do not really care either way
can you upload it to Wikimedia Commons?
Ok, does this work? File:Loose sand surface.jpg - Wikimedia Commons
yes, and I propose to add it as second very_bad sand
Anyone got low quality surface=grass_paver ?
Something that is hard to pass with regular car?
All I have seen and found on Commons are well-passable by car.
Only grass paver photo I have is only slightly worse than yours (very small misalignment between blocks).
Another suggestion for ground=bad this time with roots
A slightly different take for ground=very_horrible in case anyone thinks it interesting (messy and overgrown tree harvester track, I could take a better picture if the concept seems worthwhile).
Not sure if this counts as mud or if it’s still dirt/earth
Is the gallery intended to expand to other surface types, like wood?
It strikes me that some sufrace types cannot support the kind of vehicles listed for horrible and very_horrible, or indeed those vehicles themselves create these smoothness types…
yes! And especially this photo would be useful
can you please upload it to Commons? (maybe Commons has some images like this, I have not started hunting yet for them)
I think so, in similar way as you cannot have surface=mud smoothness=excellent
I would say that it is not mappable as a road anymore (and tagging smoothness on abandoned:highway=track has got to be an overkill)
I would say it depends on how often it is dry and how often muddy. From photo alone looks occasional rather than permanent.
If noone will post one here I guess separate thread can be made, and OSM Weekly call to action made.
Maybe this is a candidate for surface=sett, smoothness=very_bad File:IT Pompeya 0711 155 (16636268293).jpg - Wikimedia Commons ?
And this one for smoothness=horrible File:The Streets of Pompeii (14842888779).jpg - Wikimedia Commons ?
(was in Pompeii last week and couldn’t tag these roads for smoothness with StreetComplete)
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is there a chance of you uploading this to Wikimedia Commons?