Testing in VR (nice straight lines for reference) reveals that all US light vehicles seem to be affected by the direction changing issue to varying degrees. My impression is that there are two problems at play:

1. Vehicles undergo a single direction shift when changing up gears
2. Vehicles undergo a continuous drag to one side when decelerating from top speed (i.e. you go from holding shift+W to only holding W)

Also, the US M1030 motorbike seems to have a separate problem with its gear configuration. Even holding shift, on flat ground it cannot exceed 50kph, and in fact gets stuck in a cycle of reaching 50kph, changing gear and dropping down to ~45kph, changing back, accelerating to 50kph, etc.

The notes in the previous comment still apply in CSLA 1.1.

Additionally, the ADA 1600 and Sarka 1200 gained a new problem, in that they have very low artificial speed limits on certain types of terrain or road - for example, 32kph on dirt roads and a blistering 5kph on gravel roads. This feels very weird and makes it impossible to do any sort of rallying despite the addition of speed stripes.