I have a 2011 V6
with BAMA 87P tune, Airaid CAI, and 3.73 gears, and I get about 20 MPG combined. I get around 24 or so on the highway.
You have to baby it to get 31 on the highway and good luck if you're going over 70
Not sure what the EPA estimate was/is on the 2010 though.
2011 Mustang 3.7L, 6A, MCA Pkg, Ingot Silver w/ Stone
Perf Mods: FRPP 3.73 Gears, 87 Octane Performance Tune from BAMA, Airaid CAI, Magnaflow Comp Axle-backs
Ext Mods: Lower chrome billet grille, Tint
Upcoming Mods: Lowering springs