Let's start at the beginning:
15" car tires on a 15" motorcycle rim: I hope everyone realizes that this is an incompatible combination. 15" motorcycle rims are build at 15.080", while the tires are designed for a 14.968" diameter rim! That may not seem like a lot but when the beads are seated, the bead has to go over a safety hump. That increase in diameter can result in tires exploding when they seat - very dangerous.
Temporary spares (for cars) are designed to a standard with a lower margin of safety than the standard for regular car tires - as are the rims. That's the main reason for the warnings.
Further, temporary spares are manufactured to minimal quality standards - for obvious reasons.
While it it may be hard to believe, wider tires are frequently more efficient. Here's the background on that:
Barry's Tire Tech
Lastly, temporary spares are not designed nor manufactured with any consideration for fuel economy. If fuel economy is the goal, then a better bet would be to use a car tire that has been designed for fuel economy.
TodayICan: I realize this throws a monkeywrench in your planning, but I would suggest that you rethink the use of motorcycle rims with temporary car tires.