Looking through tire maker catalogs it can be confusing to see that there are 26" road tubular and 26" mtb tires but they will not fit each other rims, why?
The so called 26" road tubular tires are indeed 650c size. This means the rim diameter is 571mm
The 26" mtb size has 559mm rim diameter.
It is a 6mm difference between both size and tires will not fit on different type like a 26" mtb tire on a 650c rim, etc...