O2 Sensor question and tire question...

First question, I just did a whole exhaust on my truck and I went with a universal o2 sensor rather than oem because oem costs $220. Would your gas mileage not be as good with a universal sensor than an oem?

And my second question is, I am getting caddy rims but i don't know what tire to go with, i want to go with a 45 serier tire but what size, like 235 45 r17 be a fine size to meet the width of the rim?