what am i doing wrong?

Vegas is currently 100-110 degrees. Extremely high temperatures such as over 100 degrees will probably require you to run the air conditioner, which uses energy and thus decreases your fuel economy.

 

1) begin worrying about gas mileage on the third tank, you should be familiarizing yourself with the controls and blind spots. Aim the mirrors, learn the dash adjust the driving position, etc. These are safety measures, but gas mileage is not.

2) if you are going to change your driving in any way, work on using the brakes less.

 

It could. How far do you go in each trip? 5 miles? 20 miles? What's a typical trip for you?

 

How fast are you going on the freeway? How many times do you find yourself having to "fight traffic" on the freeway?

 

At 70-75 mph, your fuel economy will be in the 40 mpg range. With AC full blast, you can expect your fuel economy to drop a little further. If you are fighting traffic, you will probably find yourself stepping on and off the gas irradically, which uses more gas.

Slow down to 55 mph. Keep ECO mode on. Lower your AC to half instead of full blast. Pick the slow(er) lane and let people pass you if they're in a hurry.

For reference, I did a 150 mile freeway trip (light hills) with the AC at low-ish and at 55 mph and I got almost 60 mpg.

 

I am not at all sure I would have bought a city car in Vegas, but you have it now.
It does not have as good as aerodynamics as the liftback so higher speeds will effect it more. But I do 71 all day long and do about 42 in a v which is even less aerodynamic.
Do not skimp on the A/C even if it hurts MPG, the batteries are in the car with you and need to stay cool to last for a decade.

Be sure the tire are inflated. I suggest 38 in front/36 in back.
Smooth cruising, and gentle deceleration are your friends on the freeway. Ideally you never touch the brake pedal.

 

This is a liftback graph so the numbers will not be accurate, but the curve tells the speed tale:

 

Darn. I tried to tell you not to pay attention to the numbers (this is not your car) but to look at the slope as speed increases. Since your (and my) car actually have worse aerodynamics than the liftback, our slope should be even steeper.

 

Unless posted, Nevada has no set minimum speed, you are just supposed to not be dangerous.

Here in MS, when the speed limit is 70, the minimum posted is 40. I go 71 MPH personally, to match the semis. (in the v, that is about 43 MPG)

 

I lived for 14 years in Elko, you have way different weather in Vegas, but low temps hurt gas mileage, then it improves to about 85 degrees, then slopes back down above that because we use A/C. I get my very best mileage in spring and fall.

Your c is still stock, right? You have not put 'stupid big' wheels and tires or roof racks or anything, right? And you have checked the tire pressure?

 

There's really nothing to fret about. It's 100+ degrees so running the AC full blast is part of why your fuel economy is lower. (Inside the car I bet it feels like 110... I would go crazy if it got that hot)
So far, it sounds like the car is doing its job just fine.
As for the "lower speed mpg issue" - that's harder to gauge without knowing what your "typical trip" is like. Are they short trips? How much time are you spending at traffic lights? How much do you "step on the gas"? How many miles do you go per trip?

(Edit: I just thought I'd mention that there are a lot of factors that go into "city driving" so that's why I'm asking a lot of Qs. For me, my city trip economy can vary anywhere from 20s to 60s. Dealing with impatient drivers is a big factor.)

 

Never adjust the tire pressure higher than that marked on the tire itself.

Higher pressures give better MPG and handling, lower pressures give better ride.

In general, Toyota tries to choose to maximize ride comfort. Your tires should always be at LEAST as high as Toyota's spec.