He didn’t. What he said was (emphasis added)
This is pure speculation on my part but I suspect the OP is referring to averages and that his average mpg over multiple tankfulls has gone from 20 to 21.5. That’s a difference of 7.5% and certainly noticeable.
You are right. My figures are averages over about 3 yrs of driving.
This is interesting.
Yes it is but it implies your oxygen sensor has been marginal for at least three years if your mileage has remained steady. It should have decreased and then rebounded when the new sensor was installed. What prompted the replacement?
Then you might need another 3 year average to know if the sensor actually gave you an increase in mileage .
Check engine light.
Is it still on, and what are the codes?
The OP says the check engine light was the reason the oxygen sensor was replaced. I would hope it’s no longer lit.
After replacement of 02 sensor, the trouble code no longer showed.
Everything will get lower mpg in cold weather because of the thickening of the oil and the more time it takes to be efficient. Highway mpg should not be affected unless it’s a real windy day. Actually engines Run better on cold air so in the winter if you do a lot of highway driving you might see an increase of mpg