The ones I sell have a proper thermocouple and can measure any air / fluid temps up to 250°C or with the uprated thermocouple, can even be used as an EGT gauge. We also do a multiple version with rotary control, that allows multiple sensors to be connected to the same gauge (could be used to monitor transmission temps - front and rear diff plus gearbox, all off the same gauge).

In fact, we have done one for a Norwegian customer where it has five EGT probes and cycles through each one every few seconds, so that during mapping the individual cylinder EGTs can be monitored, along with the combined overall EGT in the down pipe. This means that (providing the ECU can do it), the cylinders can have the fuelling trimmed so that the EGTs are the same, which gives a good guide to the fuelling.

Also these gauges can be used to trigger water injection (with the correct relays) or an alarm feature, to alert you when a particular temperature has been reached. They also sample THREE times every second, which is sufficient refresh rate to capture rapidly changing ACTs / EGTs etc.
http://www.randbmotorsport.co.uk/shop/Parts.asp?ID=1

Unfortunately due to the high costs of these thermocouples, they are not the cheapest gauge around....