I don't think I've ever seen it tested or compared quite that way Eoth. But it's not a bad way actually, as it's pretty consistent.
But did you test the unit out of the tank, before you installed it? That's the usual recommendation here. That way you can see if it's going to be compatible with your wiring, or if it needs some friendly tweaks.
Connect it to the wiring, turn the key on and have someone watch the gauge while you run the sender through it's range of motion.
That's how I did mine, and how I found out that even the brand new factory units (I bought two from the dealer back in about '78 or so) read low on my gauge.
Took a little bending of the arm, tweaking of the stops, and I had it reading perfectly the way I wanted. Which was just above the line when full, and just below the line when sucking fumes.
When right on the empty line, I knew I had just a couple of gallons left to play with.
Because that's how my original units were reading, I ended up tweaking the originals and reusing them instead of the new ones.
It was a little tedious, because the dampened gauge does not move quickly when you run the float arm through it's paces.
And awkward as all get-out if doing it by yourself. But it can be done!
Was the original sender reading incorrectly? Or was it dead completely?
Paul