Most motor vehicle gauges provide a relative indication of operating conditions. With the exception of the ammeter, most gauges are simply voltmeters. The sensors, in turn, vary the voltage going to the gauge by increasing or decreasing resistance in response to the condition they're monitoring. The quality of ground connections going back to the battery, length of wire between the sensor and gauge, and quality of electrical connections, as well as corrosion, dirt, and oil/grease, can all affect the accuracy of the gauge's reading. Personally, I prefer both indicator lights (aka "idiot" lights) and gauges in a vehicle, instead of either/or.
