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"Narrowness", in today's tires has little to do with rolling resistance. There are 165 85 15 tires (old VW tire) with nearly twice the rolling resistance of the 185 65 15 Prius tire.
Rolling resistance varies from about .006 to .014, with sticky, sporty tires being at the high end.
The temp spare donuts are often bias ply rather than radial, and the compound is not designed for low rr -- the spare is already dangerous enough without compromising traction in favor of low rr. So even at sky high pressures the temp spare does not work well as a low rr tire. (And in every other respect besides weight, they are awful.)
There is a fair amount of science involved in designing low rr tires. The tread compound is the most obvious difference, and one that leads many people to be not all that thrilled with low rr tire handling characteristics.
There is no substitute for finding the coefficient of rolling resistance for tires. Guessing does not produce reliable answers.
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