Many "old school" operators (or maybe their mechanics) used to install a temp gauge on both heads. (one on each side of the engine compartment)
That way when there was a problem with one running warm, they could instantly tell which head it was if a head was a problem!
And as said many times a dash gauge will read OFF way off sometimes! older MCI's are notorious for bad dash grounds!
My uncle had a "9" once that if you turned on the A/C the temp gauge would go up 20* and if you turned on the headlights too it'd go up another 20* so it's be rolling down the rd @ 185-190* and I'd get calls from drivers telling me "Hey this bus is running hot!"
I'd always ask got the A/C on? They'd say yup! I'd tell them to turn it off a second and see what happened. They'd call back and say it's running 20* cooler, but still warmer than it's supposed to. I'd tell them to turn the headlights off and check it and they'd say "what am I supposed to drive around in the dark?"
One driver one night was being a real pain in the you know what so I told him "George get out and go to the back and see what the rear gauges show!"
He did and he happily reported to me "that it was running 175* now, but I know when I get back out on the road it's gonna shoot back up there!"
I told him "George if it's night time and the A/c is on it's gonna show hotter that it is.
Also I know for a fact the shut down stats are working so if it really gets hot it will shut down on you!"
He replied "So what am I supposed to do hire me some little man to ride back here an tell me when it's running hot?!?!"
I just hung up on him!