Baseline (Widescreen) Solution & Issues
Native.
Select your resolution in the launcher. Then in the in-game Options/Camera menu, turn Widescreen on to change the FOV from 4:3 to 16:9 and adjust some 2D elements accordingly as well.
Must re-enable Widescreen every time the game is started.
Game is squished in non-16:9 widescreen.
Widescreen mode has no effect on 3D models in the Garage : they stretch in widescreen.
Ultra/Super-Wide (21:9/32:9) Specific Solution & Issues
Eyefinity / Surround Specific Solution & Issues
Native, stretch (16:9 stretched to TripleHead at best). 5040x1050 is supported, it's just even more stretched than the screenshots below.
4k UHD Specific Solution & Issues
Stereoscopic-3D Solution & Issues