[brucepick]

I think the reasons discussed are all true.

What to do about it?
My gas log seems to show I'm doing a few mpg better than this time last year. I credit two things:

1) I recently uncovered the upper block and got a good rise in intake air temperature. The upper grill slots are small but apparently the intake air flows through them and thru the radiator before getting sucked into the filter box.

2) Lower grill is the big one on my Civic and I've carefully blocked off most of it with chunks of foam pipe insulation wrapped in duct tape. With that done I've been able to run some cabin heat and still get good engine temps. I watch the coolant temp on ScanGauge and I don't generally open the heater valve till its fully warm. Warmup just takes so much longer if you start heating the cabin early.