I just bought a used 2015 Prius C last week that was part of a rental fleet. One of the main reasons for buying this particular car was the gas mileage. It's rated as around 47 mpg highway and 53 mpg city. However, though I haven't driven it a lot, I've noticed that it generally only gets 20-30 mpg in the city. I didn't notice what it was on the highway.
It has been exceptionally hot where I am, so I am always blasting the A/C. Is that enough to lower the gas mileage that much? Or is this something that I should be concerned about, that maybe there's a problem with this specific car?
Toyota's Synergy Hybrid cars typically demonstrate better fuel economy in city driving because their electric motor provides most of the energy required to get the car rolling, and frequent stopping helps to recharge the batteries -- essentially giving you extra miles per gallon for free. Highway fuel economy drops slightly because once the car gets to highway speeds, the gasoline engine is providing most of the energy to keep the car rolling.
While heavy use of air conditioning can reduce fuel economy on both hybrid and regular internal combustion engines, I'd be surprised that it could cut fuel economy in half. Your own driving techniques can also contribute to your vehicle's fuel efficiency: Do you accelerate and brake hard? Jackrabbit start-and-go driving, as well as additional weight from carrying too much extra cargo, will also reduce fuel economy.
Regardless of whether you drive a hybrid- or ICE-powered car, the following tips can help you improve your fuel economy: http://www.askpatty.com/page/13_tips_to_green_your_car_and_save_gas
Since you bought the car from a rental fleet, I would assume that it came with some kind of warranty. It might be worth the time to visit your dealer to have it checked out.