The 2013 Toyota Yaris is a very fuel-efficient small car--especially if you commute mainly in a city or suburban area. For mostly highway driving, you might be better off with a compact sedan.
The Yaris has excellent 30-mpg city ratings whether you get the four-speed automatic or the five-speed manual; but its aerodynamics (and likely the lack of more gears for the automatic) keep it from returning the 40 mpg or higher ratings that other subcompacts like the Hyundai Accent now get.
To illustrate, in a test of the Yaris, we averaged 30 mpg over nearly all city stop-and-go and short trips; but at a steady 70 mph we could only average 35 mpg.
For 2013, the Yaris has mysteriously--perhaps the result of retesting--gained 1 mpg on the highway with the automatic transmission but lost 1 mpg on the highway with the manual gearbox.