80 K10 Fuel Gauge

[A-B Slinger]

1980 K10

Dual tanks. Yesterday when I started the truck the fuel gauge read slightly over 1/4 tank although I knew both tanks were full. LH or RH it reads the same. Took the truck out of town running on the LH tank and the gauge for the LH reads 1/8 after 100 miles and the RH 1/4. Refilled and LH again reads 1/4. Tried running on the RH tank and get the same readings after approx. 100 miles as I did with the LH tank. All wiring seems to be connected and in order. Any ideas ?

Thanks ahead