I believe that the SPD (speed) signal sent to the NAV unit by the Combination Meter is probably the speed signal computed from MG2's rotation sensor in the hybrid transmission (and not from the wheel rotation sensors). Since MG2 is coupled to the front wheels through fixed gearing, its rotation rate is directly proportional to the average wheel (and hence the car's) speed. And since it's sent to the NAV unit from the Combination Meter, it will almost certainly be the same data that the speedometer displays.

Stan