OP, I get 21mpg mixed(more like city than highway).
I have to drive from DU to Anschutz Medical and back 2 times a day at 8 30am and 5 pm.
You can imagine i25 then 225 or Colorado blvd at those times.
If I take it for a highway run, to Georgetown or Idaho springs to fish I get around 28mpg.
I use shell 91 mostly.
Your car might need a little tune up.
PS: I drive an 01 auto 325.
Shoot me a PM. I am here in Denver, there may be a Denver meet up soon.