I need to replace the spare tire in my 1992 944. Part of me wants to buy a full-size rim and tire, which I know will fit in the spare wheel compartment. Another part of me just wants to buy another narrow tire (155/80, 15 inches) and use the old "temporary spare" rim.

Would an ordinary, normal-duty tire mounted on the spare rim allow me to exceed the 50 mph/50 miles limit? In other words, does that limit only arise from the properties of the "temporary spare" tire, and not from the properties of the rim? Or is the spare rim not strong/durable enough to handle full speed use? (I realize that extra driving care would still be required, given the narrow spare's lesser grip.)