Playing with sierra speedos i noticed an extra circuit board attached to the P100 speedo.
I belive this is to compensate for the triangular cross section shape of the P100 fuel tank.
As far as i can see removing the board and joining the blue and the dark blue/black stripe wire would made it a standard speedo.

sierra P100 speedo (analogue clock type) back


sierra speedo (rev counter type) back

I'm no expert on these electrical matters but the sender works on a float moving up an down a resistor. The higher the fuel level the less resistance therefore the gauge reads higher. The shape of the tank makes no difference as it's the level of fuel it senses.

There again of course I maybe wrong


[Edited on 24/7/16 by LBMEFM]

the cross section is important if the sender float is linear in its output.
the P100 fuel tank has full width and length at the top (the side of a triangle)
the tank then tapers down to just width at the bottom (the vertex of a triangle)
imagine a keg/cask/barrel on its side, 10mm level change in the middle will have more volume than 10mm off the top or bottom.