The Turbomeca Makila engine uses T4 displayed in the cockpit, but this is really a 'derivative' of T3. A resistor selected during eng manufacture adjusts the cockpit ind to read whatever the manufacturer wants, according to his measurements found during testing.
P+W do a similar thing on the PT6, with their own tweek using an adjuster (NEVER used to take out gauge errors at all).
A good company has calibrated OAT inds in their aircraft, even better if they have 2 fitted. The biggest problem with the mechanical ind is the very handiness of the probe to act as a handle while somebody cleans the windscreen! When the probe is bent, the needle sticks. Most pilots I know seem to take ages to spot the stuck needle.
If we keep chatting like this, I can see pilots tut-tutting at the slightest ind problem over the next few weeks. The poor avionics guys will have to come out of the crew room.
I can hear those anorak zips going already!!