loss of mpg/ slow warmup
 

Recently after work car started right up but developed a horrible rough idle. Service light came on.. Scan revealed #8 cyl misfire.
Traced to coil pack.. Running good now, had scheduled tune up and cooling sys service..
Problem now is poor mpg. I was around 15-16 mpg in town with the orig plugs before missfire.. its dropped to about 13 after service
Mind you its been cold.. Also, seems to take longer this winter to fully warm up.. ( themrostat was replaced during tuneup)
Garage said possibly bad gas. Couple of tanks later not really any better.. no codes.

Thoughts?