What elevation are you at in Colorado?
Might be Winter fuel blends?
First change gas station and blend and see what values you get by manually calculating mileage.
Make sure your thermostat is good, see 3rd link in my signature, display coolant temp on OBC display, should be around 95C.
DISA and VANOS seals play into mileage as well.
For DISA info, see first link in my signature.