Fuel Gauge Sending Units compared

What the aftermarkets people dont understand is how these thermal instruments work. Inside the early instrument 13 inches of wire that measures approx' 1.5 ohms per inch at 68 degrees. Total resistance is approx' 20 ohms. Later instrument had 13 inches of approx' 1 ohm per inch wire for total resistance of approx' 13 ohms. Both function exactly the same. The later runs a little cooler thus lives longer.
Heat causes the bimetal to bow moving the gauge pointer. The big issue.. as that wire heats up its per inch resistance changes. Plus there is some geometry involved in the arc/bowing of the bimetal.
So from 80 ohms to 73 ohms, 7 ohms difference will lift the pointer from home to empty. Thats equal to about 1/8 of range spacing. 73 ohms to 34 ohms, about 40 ohms difference, will move the pointer to 1/4 range. 34 to 23, 11 ohms difference, moves the pointer from 1/4 to 1/2 range. 23 ohms to 14 ohms, 9 ohms difference moves the pointer to 3/4 range. 14 ohms to 10 ohms, 4 ohms difference moves the pointer from 3/4 range to full. In a nut shell , As the wire heat up, fewer ohms resistance required to generate the same amount of pointer movement. All that is why a linear sender simply will not work with a thermal instrument. Half way between 80 and 10 is what? The middle of a linear senders ohm signal. The graph plotting of OEM sender is pretty close to accurate. The plotting of aftermarket sender isn't exactly accurate but close enough to prove the fault. Maybe send this info to Specta/whoever.