yup that's true, once it gets past a certain value range the pcm will set off the light.
My rear o2 died and dropped me about 7mpg's. When my front screwed up I honestly didn't pay much attention since the light illuminated on my way to work and it was fixed the same day after work, but normally the upstream kills mpg in most vehicles.
2009 Ford Escape Limited - 3.0L, 6 speed auto, 4x4, 25/33 city/hwy imp mpg.
2017 F150 XLT-XTR - 5.0L, 4x4, 3.55's, E-locker, xtr chrome package, fx4 off-road package.
20/25 imp mpg