That is exactly what is wrong with aftermarket senders.
The tank is not rectangular, the guage is not liner either, the combination of the two makes the gauge read full for ever then 1/2 for a couple of gallons then 1/4 till it is empty. And at 1/4 it's really 1/2.
The 10 to 80 is not the issue it is the liner sweep of the sender.
I think I posted my chart earlier. About 23 ohms is 1/2 gauge the sender will show that at about 4 to 6 gallons down from full.
WORD OF CAUTION ANOUT THE GAUGES...
I have bought a number of clusters lately to get a couple good gauges to reace a toasted gauge in my cluster. I probably broke 2 maybe 3 gauges just trying to remove the speed nuts from the back
My recommendation is get 2 nuts and use them as jam nuts to hold the shaft of the gauge as you are loosening the speed nuts.
