[ijames]

If you look at MetroMPG's results table reposted above, and just compare the 155/80-13's to the 175/70-13's, the nominal height difference is within .4% (0.1" at 22.7" diameter), and both of the 155's beat both of the 175's. By an average of 20' or 11%. Even the junk 155's beat Goodyear 175's (of course, so did the snow tire 175's :-)). At constant height narrower wins for similar design tires. I don't think you can directly compare the compact spares because their construction is substantially different to achieve a similar or greater load rating in a much narrower tire; they may be better or may be worse but you couldn't predict which without testing.