Is there a reason Shadowfax is getting 16.4 MPG?
That seems extremely low...

Have you checked your air filter or o2 sensor?

Have you checked your foot?

Umm..well...we're not psychic! Tell us some stats!

Last tuneup (plugs, wires, oxygen sensor, pcv valve, air filter, etc)
Amt of city/hwy driving
Normal tire pressure
Normal alignment
type of oil used
and any other information that you think might be useful.

Well the only thing I've done to the engine in put in a FWI and that usually should imrove your gas mileage by a small margin.
I've been taking it easy on the gas since the summer because I wanted to see what I could actually get for mpg.
But what'* weird is that it was like 17.5-18 in the summer.
And it'* pretty much all city driving, which I know would have an effect but still...

it would help to know the current mileage...and what the mileage was when you got it.

That sounds about right for all city driving. Some of us lead foots would kill for 16 mpg all city...

depends on the city, if its constant stop and go, yes 16-18 is normal with any bonneville.

More curious about the strictly highway as that is a better indicator.

With the colder weather, are you letting it idle longer to warm up? I have a garage, but I always drop at least 2-3 mpg in the winter vs the summer because of the idle time. Also, the gas mix in my area changes with the season will cause lower mpg in the winter. I didn't notice it this year though - usually it occurs around October.

Remember to check the air in your tires too... mine were all low by 2-3 lbs...instant increase in mileage by 2 mpg!

Well mileage is a bit under 100000.
I got it around 87000 ish.

But I thought it was supposed to get like 20 mpg at least but if that'* how it'* supposed to be then I guess I'm ok

my bonnie is getting 21 mpgs with a mild foot in mixed highway and city. with a conservative foot and on the highway mostly, i can manage 28-33mpgs