The winter drop in FE is mainly due to the much longer engine warm up time. Once the engine has reached it's full operating temp (not just coolant temp but also oil temp) there really isn't any difference in engine related fuel economy. In my local day to day of short trips my FE drops by 15 to 20% in the winter. On the 900 drive from WA state back to California, the car returned the same 29.5mpg in December as last summer. During the December drive back the temperature ranged from 14f to 45f. This is with the same forward facing CAI as last summer.
I converted my Kjet injected mercedes 190e 2.3 to megasquirt over the summer. When I popped open the lap top again this past week for some further fine tuning of the VE table, I noticed that the car was taking a very long time to get past 160f. In MS, 160f is the last point on the cold start enrichment curve. Beyond 160f, the engine goes into closed loop and runs much more efficiently.
After a bit more driving with the laptop riding shot gun and half a dozen cold starts, I adjusted the cold start enrichment to taper down to zero at 130f and immediately go into closed loop model with the wide band O2 sensor in control.
I haven't driven it enough to gather enough data in normal driving but I expect this change will have a positive effect on winter FE.
cheers
Justin