Hi, lew

There are nots lots of solutions ...

just put 0, X, Y, Z , Full gallons in your tank ( corresponding to your Leds ) and valid Each PIC measurement with a stupid pushbutton ...

"Analog" display ( liters ) will need some more maths to find the best fit curve ...

two constants ... the non linear response AND the "12v" supply voltage influence ...

one more thing to know ... some gages output Voltage like a pot , some are variable resistors and you must create a current generator.

That's all ...

Alain