On my last tank of gas in this past week of freezing temps and only short drives for me gave me 35 mpg.
That seems to be inline with what I see; except that my commute 17mi so changes are not as drastic. I do modest grill blocking on lower grill (50-60%) but it doesn't seem to be as helpful on C as it is on Gen3. With as much you don't have to worry forgetting, as long as you don't need a/c it will be fine on long trips. BTW with 5.4mi commute even at 100% blocking it wouldn't improve your MPG much, unless you stop heating up cabin too.