fuel sending unit

I know I cant educate the world. I really should just give up. Look at this...
[ame="http://www.ebay.com/itm/Eico-1171-Resistance-decade-box-/281358605562?pt=LH_DefaultDomain_0&hash=item4182479cfa"]Eico 1171 Resistance Decade Box | eBay[/ame]
That was easier than taking a pic of my workbench this morn. Anyway...
If our gauges were designed with time piece like movement they would respond to every increment in resistance change. Then service techs would have used equipment like this to test the gauges. Fact is the gauges were not so sensitive and/or accurate.
The service techs used a 3 point tester. If the gauge responded to empty, half tank, and full it is/was considered good.
Why 20 ohm gauges are much more sensitive and accurate than the 13 ohm gauges and why 20 ohm gauges don't live as long is just too complicated to try and explain.
I'll add a pic of a 20 ohm gauge for example... You would be hard pressed to find a 13 ohm gauge that looks like this inside. They don't run as hot.
The member name Pawned and others have pics of 13 ohm gauges for comparison. they can see the difference in movement design. 20 ohm gauge shown has a pin in a slot driving a heavier needle. 13 ohm gauge needle weighs nothing and attached with a ring so its more a chain link attachment.
Even though the 13 ohm gauge lives longer, it does loose sensitivity. Test it with the factory standard 3 point tester and it will look good but by design... The sender resistance signal might go from 10 or 12 all the way to 30 before the gauge needle moves from full to half. The when the resistance signal changes from around 30 to around 45 the needle drops all the way to empty. I'm going to call the movement in the 13 ohm gauge a seesaw effect ( for lack of a better description ).