[dcb]

here is simple:

assuming you already have a speed and rpm signal on the vehicle,

program in the ratios, or "train" the cpu once at runtime (let it capture the rpm to speed ratio for the different gears).

have TWO easy access pushbuttons on the gearshifter.

When you press the left one it holds the rpm necessary to slip it into the next lower gear than you were just in.

When you press the right one it holds the rpm necessary to slip it into the next higher gear than you were just in.

If the controller doesn't see the expected rpm change immediately, it should cancel the button command and assume it is still in gear, or something, and do whatever the throttle says.