Well I've had it.
Driving in 70* temps the other day, wind and rain, ridge riding, 100% highway, but driving an equal distance into and with the wind. Tank mileage? Under 30. And I live in one of the flattest parts of North America!
Now I've got the car up to 10K Km, drive like an old lady, go 60 on the highway, drive very little city (just a few slow downs on the way to work, no stops), and have an average MPG of under 30. 28.7 to be precise based on Fuelly, after tracking over 6,000 mi. The only time I can get above 30 is by having a strong tailwind, or tailgating a semi. Air has been used very sparingly, defrost has been used very sparingly, and windows are kept closed.
I got better mileage bombing around at 70 MPH in a 95 Neon with a 3 speed auto and a 2L engine. The fuel economy is certainly not as advertised, and also not as claimed by the EPA.
Don't worry, your mileage will get better with summer gas! Also, as the motor breaks in, you'll get better mileage!
Bullpatties.
Best tank - 33.8 with changing winds that gave me tailwinds both to and from work two days on the one tank.
$0.161/mile average gas cost.
And although I put 15-20% city in my Fuelly, it's pretty much 100% highway. 1 Mi to get out of town, and 1 mi of slow down areas on the way to work.
Test driving a Fiesta at lunch today. If it works, my wife gets the Sonic as she puts on way less miles than I do.