I have a 2014 XB that was purchased new. The gas mileage SUCKS about 17-18 mpg. It has 12000 miles on it now and i figured it would get better but has not. It also has a very rough idle goes up and down sitting at a light. I talked to the dealer about it and they said that its normal for the idle and they are no adjustable. Very disappointed in a Toyota product that we purchased new and our 1999 Ford Expedition with 150k gets 1 mpg less then a new little car.

 

My wife has one, but we get 28MPG with an automatic.. It's a 2013, so basically the same car.. A lot of it is how you drive it, and keeping the Tire pressure where it is meant to be, as well as where you live. I can tell you in NY, i got 16MPG driving there, but in Ohio, i get 28MPG, because the flow of traffic is different. Where as in NY i got 252MPG in my volt, but get 196MPG in Ohio. My XB1 got 34MPG in NY, and around 28MPG in Ohio..

There are some easy tricks though..
You can actually get better MPG by doing these..
1. Get Prius tires on it, when the stock ones are shot.. Which should be soon, they were garbage.. Run them at 1-2PSI harder than recommended.
2. Wrap the stock intake in Exhaust header wrap, and use a Fram Air-Hog filter in the stock housing. The cold air and not heat soaked air from the engine will help MPG, and will make the car feel peppier.. Forget all the Tube Air filters, they don't work..

You will actually get 1-3mpg more than you get currently..
Should cost you about 30.00 for the wrap. (i believe it is a woven Kevlar)

 

We get around 25mpg with short trips around town in our Xb Mk2, 30mpg highway. The rough idle is a clue that something is wrong and it is not normal. Our idle hardly changes even when the AC kicks in.
The computer controls the air intake valve at idle (not the throttle plate) and if it is not something simple like spark plug gap, I would say you may have a vacuum leak or there is some crud in the valve that can be cleaned out with some carb cleaner.

For starters, take it to autozone and have them see if any codes are stored in the computer - it's free. If there are codes - go to another dealer, that one is a BS artist.