Not done this with a vw but done several for land rover diesels with petrol engine revcounters, usually either a divide by 4 or divide by 6 function is required, a resistor won’t do much apart from reducing the voltage, what you need to do is reduce the number of pulses per revolution that the revcounter is "seeing".
If you start by connecting your revcounter to the w terminal, use a quality strobe (needs a rpm readout if the petrol engined vehicle doesn’t have one) connected to a petrol engine.
Put a white mark on the vw crank pulley start the vw and petrol engines point the strobe at the vw pulley, bring up the revs on the vw until the white mark is stationary, read off the rpm on the strobe (or petrol tacho) and you have your engine speed, compare this to the tacho connected to the w terminal and you have you divide by factor