I'm confusing myself here which is leaner?
 

Is 950 mv leaner or 850mv leaner? I forget and want to be 100% for a new tune I am working on. Let's say my 02's are currently reading 950mv and I know from a wideband on a dyno I'm running at 13.0 afr. If I want to richen this, I would subtract from my afr to bring that 950 down to 890 or 900 right? Thanks in advance.

Lower mv, leaner. Higher mv, richer. 950mv is richer than 850mv.

If you're running 950mv, and you want to richen it, you're going to be seeing 1000 or more mv.

13.0afr is generally high 800s/low 900s on the O2s. Of course, the O2s aren't that accurate anyway.

Dope