40m tyre on 17m rims , possible or not?

[Ajax Bay]

I use the "Sheldon" guide. If you scroll down there is a table that lists rim/tyre width compatibility.

"In the interest of weight saving, most current mountain bikes have excessively narrow rims."
Bear in mind the late Sheldon Brown penned this way back and "most current mountain bikes" have 23+mm rims so can comfortably ride a 2" tyre with no adverse effect.
but why on earth is the OP rolling a steel rim: for old times' sake?