Well, that idea is truly a bust, at least with the gauge I've got. I tried both methods (math and potentiometer), hooking them up between the gauge poles, inner winding, and outer winding - and the gauge just won't register much more than enough to barely move the needle, let alone go to full travel. I did get a couple of resistors REALLY hot, though...
I think that the double-coil construction of this automotive gauge just doesn't behave like a "normal" voltmeter, probably due to the 8V startpoint .....