I though that
vstNumberOfGears = GetVehicleHandlingFloat(vehicle, “CHandlingData”, “nInitialDriveGears”)
might be very simple solution for getting accurate number of gears for every car. Unfortunately, this shows up after (…) 
and when I change to GetVehicleHandlingINT, this
Edit.: At least, in the last case, I am somehow able to shift up to “actual” highest gear of each car, even though that insanely high value above messed up the whole calculation for min and max speed for each gear, which means I can’t move, because the car is limited to 0.00023 km/h…