X35D thermostat, maint and musings

[FellowsheepTech]

So...
I replaced thermostats a couple of weeks ago & twice had a lot of coolant leak overnight out of the quick-disconnect fitting on the front bottom passenger side (the point many have recommended for draining the system). In all the threads I read through, I never saw this mentioned but the fitting seems prone to this? If it happens again I'm replacing the fitting with a flush kit.

Checking temps, it's now operating as expected, but just returned from a 700mi trip & was very surprised/disappointed to have LOST 3-4 MPG (down to 23ish from 27ish). I drive hard, and often go back & forth from NAS Pax area (SOMD) to Binghamton, NY...so some mountains involved (still hard for me to say that having grown up in Colo.). Factors:

1. I'm sure the primary factor is upgrade from std. 18" stock wheels & worn tires to 20" M-sport staggered wheels with nearly-new General Lexani tires. Much wider, but sized for these X5s and typically new tires would give better mileage than old. These came off a 2009 3.0i X5, so I assumed they were correct size/circumference, but just ran them through online calculator & it shows approx 1.3% difference. Is there a way to correct the computer for tire circumference?

2. I've now put 7kmi on the truck, and got a DEF low warning for the first time. I was told it was topped when I bought it, & I've never added, but have noted on scanner that regenerations were happening, though didn't analyze frequency. I would expect better regen with higher engine/coolant temps, but it was regenerating before; I truly have no idea how much DEF I should expect to use? I wish there were a way to read the level!

As an aside, why fill both passive & active sides if the passive feeds the active side?

Now we've got to go fix the aftermarket lights on my son's 540...