Plenty of good advice already, especially BC's & Whynot's comments.
All I can add is that "warming it up" before driving will REDUCE the MPG, not improve it!
And that I've seen a typical 5 mpg drop in mileage from winter tires instead of high mpg summer tires alone, e.g. - a vehicle that was consistent at 30 mpg summer went to 25 mpg with winter tires with no changes in driving BEFORE the weather got colder & winter fuel was involved.
Tires & 2.9 mile drives will give these results, my Focus that gets about 28/29 in "mixed" driving has gotten as low as 18 in strictly local driving (short trips only) and that was on summer tires... Same car will get high 30's on the road (highway only).
Do all the maint. required for the best results possible of course, but it seems like you're just dealing with a "worst case scenario" of real world driving....
An OLD hint for cars subject to this type of use is to take it out for longer trips as often as possible, the proverbial Sunday Drive is actually good for it! Lets it get warm enough, long enough to clean out some of the condensation & other crud that doesn't get a chance to evaporate in daily use, and even 'burns out the carbon" a bit. Modern engines with electronic fuel injection aren't as bad in this regard as old carbureted cars in this regard, but it still helps....