both the speed sensor and the fuel sending unit are just two different ways to measure current. The speed sensor is a magnetic switch, every time the wheel makes a complete revolution the switch triggers to sends a shot up the wire. That switch needs a power source ,one wire in and a signal wire out to the speedo look at the back of the speedo. there needs to be a ground and a incoming/ signal wire connection. Most speedo's have four wires. A ground wire a hot to battery, a wire to the sensor and a return from sensor. The wires to the sensor would have to be tested with a meter as one will stay continually hot to the sensor while the return would be interrupted with each revolution.
The fuel sending unit is nothing more than a mechanical sliding resistor The float changes the amount of resistance. The gauge is nothing but a amp meter. It reads the volt/amps coming from the sending unit. If you divide the face into 12's instead of full/empty, it would tell you the output voltage of the sending unit This is wired the same as the speedo. A constant hot to the sensor, A return with a fluctuating current. Again you would need to use a meter to find which wire has the fluctuating current by simply moving the float. With the speedo I don't think it makes any difference as to which wire as it is only a momentary on/off that interrupts the current flow. it could make a difference with the fuel as the gauge may read incorrect. As long as you know which wire does what you will be able to figure out the correct connections.
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