This has been known for a while and the community asked for it previously. Hopefully if we keep the pressure it will be fixed for Mudrunner (aka we shouldn't have to wait for Mudrunner2 to get a working automatic transmission since it's part of the core gameplay).
If finding a correct algorithm to always downshift to the right gear would be difficult then I would propose a simpler one : when a downshift is to occur simply downshift two gears, except for when we're in second gear in which case it should downshift to 1.
- gear 1 -> no downshift;
- gear 2 -> downshift to 1;
- gear 3 -> downshift to 1;
- gear 4 -> downshift to 2;
- gear 5 -> downshift to 3;
Provided that the gear ratios are properly spaced and that a right downshifting point can be found then this should work rather well. It should also work well for modded vehicles with more than 5 gears with maybe the modification that starting with gear 5 the downshift should cover three gears.
- gear 6 -> downshift to 3;
- gear 7 -> downshift to 4;
- gear 8 -> downshift to 5;
- gear n -> downshift to gear (n - 3).