Karin,
'600 miles'?! How many tanks could you have done the arith with, in those few miles?
It's brand new, tight and I wouldn't consider worrying about it until you have a couple thousand on it...if you are still getting '13', then something is amiss.
The OBC readout will probably never be on the actual tank math, and imo, the current EPA 'test', with 'adjustments', provides for much higher potential 'hwy mpg' than anyone I know, including me, gets on newer cars, vs the older EPA standards.
As for driving like Grandma, it's your call; I'm not on the bench with the "drive it like you stole it" group, but some speed and rpm variation is good for a new car, imo.
My 50Cts...
GL, mD