why does my 2005 sienna awd get 12 mpg in the city, 17 mpg on the freeway?

The mileage that dealers and auto makers post on a car are figured based on someone driving 55 mph on a flat road. If you go over 55, you drastically reduce your fuel efficiency. If you are doing 12 MPG in the city , then you must do a lot of stops and starts

I have the same problem with a 2006. I've been fighting it ever since buying the damn thing certified pre-owned. I have found that if I use 91 octane and fill up when the tank is around half empty I can get 19 to 20 city with around 26 highway... not great but better than the 13/19 I was getting before on lower octane. Filling up after using only around 10 gallons is something I haven't been able to figure out. I have been able to determine that if I fill up less than or greater than when half the tank has been used I get the same bad mpg as on lower octane.

My 2006 Sienna limited AWD gets about 14 mpg overall and 20 mpg on the highway. Mine gets better gas mileage at higher speeds. During the trip from DC to FL, I drove at over 70 MPH and got just about 20 MPG. At home, everyday driving yields about 14 MPG. Lower than the sticker when I bought it new 4 years ago. I put in regular gas mostly from Exxon or a major vendor (Shell, BP, Chevron, etc) never from the Crown or a mom and pop shop (no brand).