Hi Roger. I looked up the calibration for my 1936 gauge, which is of the G33 "Sender is 25 ohms when empty" type. The required sender resistances and corresponding sender currents with a 6v battery connected were as folllows: MAX is 0.3 ohms/297 mA, 5 gall is 1.6 ohms/247 mA, 4 gall is 3.4 ohms/206 mA, 3 gall is 7 ohms/155 mA, 2 gall is 9.3 ohms/134 mA, 1 gall is 14 ohms/105 mA, 0 gall is 20.7 ohms/80 mA and minimum is 26.8 ohms/66 mA. The gauge internals are remarkably ingenious, with the needle pulled in two opposite directions by a tug-of-war team comprising two coils with magnetic pole pieces. As the sender resistance drops, the current in the "T to ground" coil goes down and the current in the "B to T" coil goes up. I guess this ratio arrangement makes it immune to a certain extent to battery voltage (but not for use directly on 12v !). From memory the coil winding wire is silk or cotton insulated, so shorted turns are not impossible after nearly 80 years.

Location: New Forest

John. Thank you for those useful figures. It's too soon for me to know how the bodged original gauge will read after adding the 25 ohm resistor but early signs are that it will fall quickly from the "Full" and head for empty faster than it should. I can probably live with that kind of error as it's safer than the other way around.

However, being a heretic as far as total originality is concerned I've already bench tested a simple circuit that takes a steady 140ma and uses a modern 1 ma meter movement. I think It will fit inside the old case if it becomes necessary. With those resistance readings I can roughly predict the linearity when connected to the sender. Probably too much detail but I found a VU meter which displays upside down (ideal) and has a red section around what would be the low fuel end.

The original fuel gauge of that period is indeed ingenious. Like the equally ingenious cutout it was designed when electronics was in its infancy. The gauge copes with the range of voltages that a 6v battery exhibits from discharge to being charged. Roughly 6 to 7 volts but probably it betters that.

My replacement, if it ever gets used, uses a voltage stabiliser as I believe all post war cars did. I took the tiny pcb from inside an old mobile phone in-car charger (the size of a 10p piece)to get 7.5v. The car has to be 12v though to make use of those. With apologies to the mechanics for talking dirty
getting good with the captcha..

Location: A little East of Sandy