Mine is exactly the same way. When full, the gauge shows emtpy for the first 20 or 30 miles after a fill-up. Then it wanders around for awhile and settles to start reading more-or-less correctly somewhere above 3/4 of a tank. I've never run it until lthe light comes on, guess I should try, just to see if it is working. But reading 1/8 of a tank it will only take 20 or so gallons to fill. So the gauge is obviously way inaccurate, but at least it is on the conservative side, so no real issue for me.

 

There seems to be only one quirk to my guage and that is that it never shows a full tank just after I fill it no matter how full it really is. It takes a day or two for it to get the message and eventually read just above the full mark. I used to think it was accurate and I would try to fill it with the front end down or up. Didn't matter. Just some Dodgy electrics. It probably is about 30 of your tiny little US gallons. I think that equates to about 24 BIG imperial gallons. My chev had a 28 imperial gallon tank and took around 130 litres on a normal, kinda empty fill. Ironically, my V/6 chev (longbox and standard cab) had room for the big tank and was so equipped. My buddy had a 3/4 ton, extended cab,low geared, 454 powered, automatic, 4X4 he used to pull a RV trailer and it had the small tank due to the shortbox on the extended cab. My tall-geared pickup could go 1000 km on a tank, he got 300 to 400 km at best.

I got an idea, why not a sight glass like a coffee urn?

 

when i got my truck, the PO. would just reset the trip meter every 350 miles when he refilled(allso had a 100 gal.tank in the bed)
this drove me nuts, so i pulled the tank and replace the sender!
now when the low fuel light comes on and stays on. it take 27 gal.

 

Your experiences are similar to mine. When pulling I know I can go about 50 miles when the light stays on and about 75 or so bobtailing. That means it has about 3 to 4 gallons left when the light comes on.