My two cents on observed mileage
 

So I just moved to another place farther out from my office and now the commute is over highways mostly and no stop lights.
My average mph has gone from a struggle to stay above 12 in the traffic before to now 17.9 and I'm sure it would bust over 18 if I reset the computer.
The engine was having trouble staying "lit" at stop lights and stumbling on launch, but it seems smoother now.

Maybe it always hated the stop lights and traffic and is feeling better now.

Did you mean MPG instead of MPH?
If you are only managing 18mph you may as well get a bicycle.:)

On my old 96 XJ6 4.0 l my average was always around 28 mpg with mixed driving. You might want to investigate where you fuel is going.