Originally Posted by
alan s
... as long as you keep the tire pressure at around 75% max pressure rating.
Consider this a quibble rather than a correction.
The rim neither knows or cares about the tire's pressure rating. It only cares about the actual hoop stress the tire pressure causes. Rather than focus on the pressure rating of the tire, which can vary tremendously tire to tire of the same width, consider the high end of the spectrum of pressure ratings of tires the rim maker would expect you to use. (ie. the pressure ratings of the tires in the center of the charts.
Then adjust that based on the width ratio of the tire you're using to the rated tire to find the maximum pressure from the rim's point of view. Subject to the tire's rating, this would be the max. leaving the same margin for safety as would the narrower tire. Of course you're free to use lower pressure balancing ride and handling with comfort.
By example, if the rim is expected to handle a 25mm tire at 140psi, it could equally handle a 50mm tire at 70psi, not that you'd want that.