Cheers birchy, yes the car voltage will be 14.4v so i have used your link to calculate that i need a 620k and 20k Resistor if the voltage is 14.4v and the output was say 0.45v

As for getting my supply voltage there is normally 4 wires to an O2 sensor and two are the heater wires that are the voltage of the car i.e 14.4v when running, i will tap into this supply. looking at the schematic on that Potential Drive Calc will the resistors get hot or blow if i wire them like it suggests and take my feed from between the two?