I don't know what you mean by "mechanical" gage but if it has a D'arsonval movment don't use it. You need to be real careful what you hook up to the lambda signal because it can be easily effected. Whatever you connect to it must have a hihg impedance and mechanical gages don't. In fact I know of at least two electronic A/F gages on that market that'll attenuate the O2 sensor signal.