a 47 ohm resistor in parallel with a 90 ohm resistor results in a 30 ohm resistor. So putting a fixed 47 ohm resistor across a 0-90 ohm sending unit will effectively change it to a 0-30 ohm sending unit. There will be some non-linearity in the gauge caused by this, but you can ignore that.

so simply solder the 47 ohm resistor across the end terminals of sending unit resistor