You've got a contradiction, good gas mileage and an automatic--they just don't go together. Learn to drive a stick and save.
In this day almost all cars get as good mileage with a stick or an automatic.
Not enough of a difference to worry about for most peoples' driving habits.
My advice is to buy a low mileage used car. Why should you take the initial depreciation hit when you can let someone else do that for you.
Also, right now, you can buy a lot of small, high mileage cars that cost FAR less than $20K.
Look at Edmunds.com for some help.
I have a Dodge Caliber and it fits your requirements to a "tee". Great little car, and gets about 30 mpg.
I have a manual, but they're harder to find, so an automatic transmission is what you'll probably find at most dealerships.
Buy a motorcycle.....good visibility, and good mileage, And you save money on windshield wipers. a little cold in the winter time..
Buy a motorcycle.....good visibility, and good mileage, And you save money on windshield wipers. a little cold in the winter time.. 
I second that, as long is it isn't a crotch rocket. Nobody needs to go 180 mph. My last bike got 50 mpg.
If i'm not mistaken SaintC, a couple of the CVT transmissions from Nissan get better mileage than their stick counterparts.
Yeah dog, I totally concur with Mike, it doesn't make a whole lot of sense to buy a new car when you can dodge initial depreciation. My personal pick for your points is a 2 to 3 year old Hyundai Sonata. Korean vehicles suffer from some of the worst depreciation in the business because of the vehicles they made 20 years ago. The market has not corrected for the fact that the cars they offer today are fine and completely reliable vehicles, on par with their japanese counterparts. I picked the Sonata because it's a big ride compared to your civics and corollas that your friends above 5 ft 8 can fit in the backseat comfortably while offering 21/32 mpgs. Don't go earlier than 06 as that's when build quality becomes more of a risky proposition. A 2006 with 30k miles should go for about 10k, but i'm sure you can find a deal if you sit on the papers and craigslist long enough.
As Mike pointed out, Edmunds.com is an invaluable tool. I was an edmunds junky before coming to SBG and it wouldn't be uncommon for me to spend 2 hours a day reading all their vehicle info. Great site.
The biggest mistake you could make is settling for a new subcompact that you don't enjoy driving everytime you hit it up, when you could spend the same amount of cash or even less on a more comfortable car with only 10 to 30 thousand miles on it.
Young Casey is wise way beyond his years..... Good job Grasshopper!