I have found cool temps affect FE even on long highway trips. I recall taking a summer trip last year and noted that during long gentle down hill runs at about 65mph (on cruise control) my FE meter would reach fuel cut (0L/100km or 60mpg) and I got an indicated 5.6 (42mpg), but taking the same trip at the same speed this time of year I was not able to reach fuel cut, there was always some fuel being used. It gives me about what you got, mid to high 30s.

I also notice that even if start and end points are the same elevation, FE will be affected by the duration of downhill runs. If you get long gentle downhills with steeper shorter uphills that is great for FE. But coming back, long gentle uphills with steeper short downhills kill FE.

Two climate control systems, one inside and the other at the tailpipe.

2007 Camry Hybrid (in service June 2006)
FE: 6.77L/100km (34.8 mpg) over 37750km (23450mi)
75% city - Ave trip 20 min - 2 Winters