This is what is puzzling to me. It's only the 25MPH Nox reading that is off. The 15MPH Nox reading is fine. The only difference running-wise is the RPM change of a little under 1000RPM, unless there is some component I'm not aware of that takes MPH into consideration and/or the computer does something weird at that speed. I was hoping there was something obvious that I've overlooked and that someone has run into that situation before who knows what it is.
You say you have no codes like a p0441. Does it have evap? Is it four-wheel drive and could there be a crack in a vacuum line that goes down around the transmission to some hard lines into the front axle. EGR stuck open not closing all the way?
It would be a bummer if a crack on the exhaust seat made it to coolant that'll cause a misfire, otherwise it could be cracked into the intake seat as long as it doesn't have coolant leaking through keep running and not bring any cells