I also average at best 30 US MPG in city driving (25 only for the first thousands miles), and above 40 MPG on highway; not such a difference with bigger cars.
The Yaris got a bad review on Consumer Reports. The Honda Fit is rated much higher.
Every car I buy I immediately switch to full synthetic oil, a K&N air filter, and in some cases a more free-flowing exhaust. These small fixes cannot only boost your mileage, but might also give you a very small, but noticeable increase in power, which the Yaris could use. On a smaller car such as the Yaris small power boosts can be very noticeable, and the extra mileage will pay for the minor changes in a year or two of driving.
K&N filters will ruin your MAF sensors. Oil from the filters get trapped in the MAF and if you never clean it (the MAF), it will fail.
I own the very same car, with automatic transmission, and routinely get 33mpg in the city and 40+ on the highway. Definitely check your tire pressures -- also pay attention to your driving habits -- average speeds and the way you accelerate from stoplights. If it's not your driving style, it may be something your dealer needs to look at.
To the reviewer and to others who even consider trading in a vehicle: You'd be better off donating a used car to charity because dealers do not give you anything for them!
Thanks Mr Honda Car Salesman. This is an awesome little car if you want to leave a tiny footprint. I consistently get 45-48 MPG on an imperial gallon, driving highway and city combined. A little less in the winter. I spend $50 a week compared to my old Sunbird, which costs me twice that. I figure after 5 years of driving this, it will have saved me $6k. It has 150k now and still has its old brakes. Oil changes, 2 air filters, a timing belt, and 8 tires is all it cost me... Nuff said!