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There is very little change in rolling resistance due to tire size. The big differences are between tires - meaning make and model.
In the 2 sizes you are considering, there is less than a 1% difference in rolling diameter, so you won't get any better fuel economy due to the gearing effect.
So I would stick with the 175/70R13 as there are more tires available - although this size isn't very popular anymore.
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