If it was a typical emergency spare (low diameter) they are specifically prohibited from running even at 55 mph. (40 miles max distance I believe.) Much less 75!
If it were a cobbled replacement (regular tire was flat, another “ordinary tire” substituted) it could be any size, any mismatch as well.
Most trucks have a full size spare, which likely had been tucked up under the truck since day one. The southwest is tough on tires. I’ve had ones start to fail after 3 years being outside in the sun. Dry rot, tread separation, not uncommon.
Hard to imagine a Ram 2500 with a little temporary spare.