Look, I love my Sonic and think that yes, you should get one, and that you'll love it.
But if you're looking to save money, you should really sit down and do the math.
You own the jeep free and clear, so your only expense is keeping it running.
Your commute is... lets say for easy math it's 15 miles, or 30 miles per day, 5 day a week.
In the jeep, you're going through 2 gallon per day. If your Sonic gets 30 mpg, you'll go through one gallon per day.
50 weeks per year, 5 days per week, you'll save exactly 250 gallons. At (lets say things get worse, just so the math is simple) $4 per gallon, that's $1000 saved per year.
In 15 years the car has paid for itself...
Also, your commute is too short to get good mpg, startup and the first several minutes of driving are all bad on mileage. I'm not saying you can't average 30mpg... but you'll do it by getting 20mpg half the trip and 40 mpg half the trip. Except... those highway miles, are we talking 60mph highways, or 80mph interstates? Because "highway" is really going 60ish, as your speed goes up over that, efficiency drops.
Too, cold weather will reduce your mpg, and hot weather will too (assuming you run the AC in hot weather).
You've selected the best make and model for maximum mpg, the LT is the lightest Sonic with the turbo engine, which allows you to drive gently and reap the most rewards (it also allows you to drive harder and get Worse mpg)
But the biggest factor in your mpg is your own driving habits. I normally drive somewhat senselessly, accelerating hard from stops, exceeding the speed limit by a good amount. In a test last spring, I drove an entire tank as I normally do, then went back and drove the same route, which was 80% highway 20% stop and go city, sensibly, and increased my mpg by a whopping 50%. That's not driving under the speed limit, not accelerating so slowly that folks were piling up behind me on the on ramps or at lights, just taking it easy and staying in the right hand lane. There are people here reliably getting over 50mpg... they are doing a much better job of driving gently than I will ever do.