No, the entire circuit is 0-5v from the instrument regulator to ground ... the regulator limits potential and the sensor further limits the voltage on the circuit. (I know this doesn't seem to make sense but it's what happens ... and is part of what made me quit studying analog circuits.)
The potential is 12v on the ground side of the sensor and the supply side of the regulator. Between the regulator and the gauge, it should be around +5v. Between the gauge and the sensor, it should be at or somewhere below +5v.
The relationship between engine temp, sensor resistance, voltage, and needle position is not linear. So, the voltage at operating temperature is not necessarily +2.5v. As long as it's somewhere between zero and +5v, the circuit is working correctly. (The gauge is designed to compensate for the non-linear voltage.)
