This is the speedo output signal (not the VSS) I assume. The pull-up resistor limits the current to the external device (i.e. Abbott cable + convertor in your case).
Are you using a 5 Volt or 12 Volt pull-up?
With a 5 Volt pull-up, you need to limit the current to the device, but you also need to limit the current drawn from the GPIO's regulator to less than about 50 milliAmps. So I would use no less than R = V/I = 5/0.050 = 100 Ohms.
If you are using a 12 Volt pullup, you probably don't want more than about 200 milliAmps (to avoid stressing the power supply on the board), so the minimum resistor would be R = 12/0.200 = 60 Ohms.
You might want to put a weak pull-down on the signal as well (to pull it to zero volts when the signal is off). A 10K Ohm to 100K Ohm resistor on the signal to ground (this can be done on the board or externally) should work.
You might also want to experiment with the duty cycle on the speedo output, as some external devices can be quite sensitive to this parameter. See: http://www.msgpio.com/manuals/mshift/V4tune.html#su