It's an Autometer mechanical (capillary) gauge. Can't remember if I tested it in boiling water or not (think I did a long time ago). From the underhood heat and touching various cooling system parts, I think it's reading about right. I agree, 198 maximum is not too hot as long as it doesn't keep climbing, and I've got lots of oil pressure. But I would rather have it cooler as protection from detonation on pump gas. And the thermostat ideally would be the sole determinant of operating temperature...

Ambient temp varies a lot here, of course, but it's been doing this since I got it running four or five months ago (50F or 95F, same behavior).