I prefer to replace the temporary spare with a complete spare wheel with a regular size spare tire - most any Volvo rim will work nicely. On a 740T you'll need to check clearance if trying to use a 14-inch rim (as from an older 740GLE or a 240), but any wheel from an early car (75 or older) or from a base-model 740/940 will be 15-inch, and clear all the brakes perfectly.
A full size, normal tire is much safer - you can complete your trip at a regular speed, without any handling irregularities.
We usually put the extra space-saver tires on the cars we send to the junkyard, and keep the stock rims.
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Rob Bareiss, New London, CT ::: 86 244DL, 87 244DL, 88 744GLE