Universal O2 Sensor and tire question

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Over in the "Introduce Yourself" forum, Ryland suggested that I put different tires on my 92 Civic CX and Matt Timion suggested a new O2 sensor.

Can anyone help me to understand why the 155/80 tires would be better? As Ryland guessed, I do have the 175/70 tires. I'm new to understanding how everything affects mpg -- any tutoring/apprenticeship would be happily received...

Also, I'm looking into getting a new O2 sensor at Matt's suggestion. The CX has 186k miles and looks like not much has been replaced. Rock Auto has a Bosch OE type for $205 and a universal for $21. Any reason not to go with the universal? I've got a relative who is an excellent mechanic and especially skilled with electrical components who could help me with the universal.

By the way, I'm having great fun driving the CX and got 37.5 mpg on my first tank (with a flat tire in the trunk, using AC, and old O2 sensor).