I live in DC and drove to Alabama this holiday weekend. my GF suggested I use High Octane and get better milage....so I did. But i wanted a good comparison...so I drove off with 1/2 of a tank of Reg. I refilled with Reg again and wrote down the milage. Short story:

On the way down:
1st fill up 73125-73559=484 I got as close to empty as I could. Filled up with 1.5 gallons. REG
2nd Fill up: 73559-74002=493 REG this is part of the way down....didn't drive while there.
Way up:
1st fill up 74002-74545 (74510 Exactly 2 gall left so I added the 35 miles to empty to the total...could be more could be less)= 543 Premium Unleaded 18 gal/543=30.1 gived or take a few

I have currently a lil less than 3/4 to a 1/2 of a tank...got caught in traffic. Total cost round trip was $218.00...

And we hit speeds of 85-90 at times...she has a lead foot!!