Yup, in my hot water testing of thermostats, all the US ones start to open at their listed temp and are fully open at another 15 or so degrees.
Every parts book in the US I've seen lists the wrong temp thermostat for D1's.

 

So it is the double whammy on the older trucks with multiple owners.... missed maintenance, then running warm, and the cheapest thing to change out is the $10 stat, and then the darn thing runs hotter than ever, now at a slightly elevated temperature, while the "wonder gauge" from the factory shows normal range. Probably all thanks to some computer weasel in a cubical somewhere, who was retired after the update from DOS 5.....