I've had an ongoing issue with peculiar high temps on my GTA turbo for years now, all following an incident where the car lost some coolant due to a split hose. Even though the gauge didn't even hit the red sector it simply never read the same after that, and I've spent lots of money and time over it - replacing the thermostat, temp gauge, sender unit, rad etc plus even more costly items!
As it is the car will still read mid way between the 90 and the red on hot days under load. I finally decided that the only way to be 100% certain what was going on was to make up a temporary gauge and test it out.
Low and behold the gauge always over-reads and my digital thermometer peaks at exactly 90/91 degrees so far even when it was 30+ outside.
Don't laugh too much but I used a digital meat thermometer used for checking your Sunday lunch.
(It was cheap and I could vandalise it). I snapped the probe down to the correct length and silicon sealed it into a brass union (bought for a few pennies from B&Q plumbing section), you can then screw it into the water pump body in place of the normal temp sender unit. See the pic below for the probe and temp unit.
I could then drive around and get live temp readings (I taped the digital readout to the inside of the small rear glass). At the moment I've got the digital sender taped to the side of the dash sender so I can calibrate the dash gauge against real temp - and it really isn't accurate. So it goes nicely with the speedo then
Anyway if anyone wants any more info on checking if their engine is 'well done' or just 'medium rare', then let me know!