LMU Graphics Best Settings

Video Device: 6900XT - you also want to underwolt this one or possible other AMDs.
My case: 7680x1440@120hz on X11 (6900XT with Ryzen 9800X3D) (GPU offset -81 via LACT app)
Steam launch: mangohud gamemoderun %command%

You want a 9800X3D for LMU, it is a beast, I upgraded from a 5800X3D - It's like 50% better, LMU is Single-Core CPU bound 
most the time.

Before Starting the game, get the proper frametime for your resolution and desired FPS with cvt (7680x1440@120fps my case):
slemke@gamesmachine:~> cvt 7680 1400 120
# 7680x1400 119.92 Hz (CVT) hsync: 180.24 kHz; pclk: 1929.25 MHz
Modeline "7680x1400_120.00"  1929.25  7680 8336 9192 10704  1400 1403 1413 1503 -hsync +vsync
slemke@gamesmachine:~> 

And set it in the config file:
slemke@gamesmachine:~> grep 119.9 ~/.local/share/Steam/steamapps/common/Le\ Mans\ Ultimate/UserData/player/Settings.JSON 
    "Max Framerate": 119.92,
slemke@gamesmachine:~> 

Start game, and game settings:
Vertical Sync: Off
Fullscreen (Important on KDE on single monitor, otherwise kwin keeps interfering, windowed for triples.)
Post processing: Low
FSAA: MSAA 4x
FXAA: OFF
MultiView: OFF (ON for Triples)
Display VR Mirror: OFF

(Ultra VRAM 16G is not enough on big circuits)
Circuit Detail: High
Texture Detail: High
Player Detail: High
Opponent Detail: High 

Texture Filter: 8x
Texture Streaming: On

Special Effects: Mid
Shadows: OFF #IMO shadows in LMU look horrendous anyway.
Shadow Blur: OFF
Soft Particles: OFF - Gimmicks
Rain Drops: Medium
Road Reflection: Low
Environment Reflection: Low

Visible Cars: 20 if 9800X3D, other cpus better 12.

#A full cache cleaner, you may run it after doing bigger graphical changes:
slemke@gamingpc:~> cat clear-cache.sh 
#!/bin/bash 

#Clear Steam's global shader cache
rm -rf ~/.local/share/Steam/shadercache/*

# Clear a specific game's shader cache (replace <AppID>)
rm -rf ~/.local/share/Steam/steamapps/shadercache/*

# Clear DXVK cache for a Proton game
rm -rf ~/.local/share/Steam/steamapps/compatdata/2399420/pfx/drive_c/users/steamuser/AppData/Local/dxvk

# Clear Mesa OpenGL cache
rm -rf ~/.cache/mesa_shader_cache/
rm -rf ~/.cache/radv_builtin_shaders

rm -rf ~/.steam/steam/steamapps/common/"Le Mans Ultimate"/UserData/Log/Shaders
rm -rf ~/.steam/steam/steamapps/common/"Le Mans Ultimate"/UserData/Log/trace*

#Remove practice sessions from replays
rm -rf ~/.steam/steam/steamapps/common/"Le Mans Ultimate"/UserData/Replays/*P1*