I have a manual shifter with a clutch. I just noticed that even with the clutch pressed you get engine braking (for example when going downhill) if the shifter is in gear. The game should ignore the gear if the clutch is pressed and act like the truck is in neutral.
Additionally it would be more realistic if the vehicle didn't immediately slow down to the maximum gear speed when downshifting and if we could hear the engine rev up when doing so. I know this part isn't really a bug, just the way it's implemented, so this part is a suggestion.