Instrument Cluster

You can do better than that for testing. Chrysler used to make a gauge test tool, which is basically 3 resistors in a box, to simulate MT or low, middle of gauge, and hot or full. The resistors are + /- a few ohms

L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)

EG for middle, you (used to) be able to go to Radio **** and buy a pack of 4 100 ohm 1/2 watt resistors. Wire all 4 in parallel, this makes 25 ohms. Connected from sender terminal of gauge to ground, this should give you 1/2 scale on the gauge. Leave it a couple minutes to settle down, and hold the cluster "as if" it were angled in the vehicle.