Switching depending on resistance from fuel gauge sender unit
You are looking for a DC voltage monitoring relay.



Figure 1. A random voltage monitoring relay.

Something like that of Figure 1 has some nice features. Without reading the datasheet I would guess that it has adjustable on / off delay, hysteresis (the difference between the on and off switching points) and a U-W function - whatever that is!

If it's still on the original circuit you can simply monitor the output with a monitoring relay. If the sender is not in-circuit already then you'll have to feed a safe current through it to generate the varying voltage for the relay.

Take care that the device is explosion proof as required.