I have Schwalbe Marathon Winter tires on now, 26x1.75, on Rigida Andra 30 rims with 19 mm inner well width. I measured the width as 1.75.

Tricky to get really precise with such a measurement! I improvised some crude calipers: I have a carpenter's square, and then I took a magazine and kept one edge flush with an other edge of the square. The spine of the magazine was thus parallel to the other edges of the square, with a gap between the magazine spine and an inner edge of the square. I could slide the magazine, keeping its bottom edge flush with the bottom outer edge of the square, until it was snug against the tire, the inner edge of the square snug against the other side of the tire.