Hello once again,
After some further more testing and after later game updates of Mudrunner (these numbers are for Mudrunner, but could very well give similar results on Spintires), I had to re-tweak the values a little bit more. Now the mud grip is slightly lower and you may get stuck in the mud more often if you don't have something to winch onto or another truck nearby for help, but the purpose remains the same: realism as better as the actual physical model simulation allows.
Here it is:
The same steps are needed:
Extract the content in a temporary place.
Overwrite that extracted content into the "media.zip" file that is found in the game's folder.
Cheers and regards,
-I wish that the developers would try to use similar values (such as the ones that I've tested) as default. I'm pretty sure that a lot more hearts will join this unique game/sim after seeing more realistic physics!