I hate to say this, but having the wheel sensors and gyro improve the accuracy

It allows the Navi Computer to continue calculating vehicle position during moments of GPS signal loss.

Just as a thought, have you tried using the nav option to reposition the pointer - I cannot recall exactly which option it is (you get at it from map mode if I remember correctly), but it allows you to set the pointer at say a known road junction, and then when you drive over that point, you press the command knob and it supposedly latches your vehicle position to the pointer position.

I used to have to keep doing this when I had the wheel sensor problem, but it does drift out quite quickly.