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The difference between tires in different sizes - all other things being equal - is small compared to the differences between make and models (up to 60%)
So you should concentrate on the make and model first, before you tackle the size thing.
But to answer your question, a 165/80R14 is 7% better for RR than a 165/60R14, while is 155/80R14 is only 5% better) - but I doubt you can fit a 165/80R14 under the fenders of a Corolla.
But the above doesn't take into account the affect the rolling diameter will have on engine performance. I don't think you can quantify that to any degree of precision, especially given that we don't know how you use the vehicle. Lots of stop and go driving consumes a lot of fuel compared to the distance traveled, so any improvement a different tire will give you is going to be small compared to the weight being accelerated.
Also, you've posted a similar question at Bob is the Oil Guy.com. You're sort of getting the same answers there.
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