I buy gas all over the spectrum of name brand and off brand stations, and have yet to have it impact my miles per gallon. Driving 75 MPH certainly will affect the mileage, however. I drive 80 miles a day through the heart of Denver and it's traffic, and my last tank was just under 54 MPG. All I do different is keep it to 62 MPH on the interstate.

AC use can have a huge impact on mileage. We set ours on 74, and have lost about 5 MPG since the hot weather set in.
If you want 50+mpg, you need tires at 44 PSI, slow starts--avoid over 2500 RPM where possible, coast as much as you can, minimize use of brake, including regenerative braking, and don't go faster than 60 MPH ever! These tricks work on any car, BTW, Suburban or Hummer as well as an HCH or Prius
But keep in mind, driving like you do, you would get 20% less than EPA ratings in any car, but not many cars tell you up front like the HCH does.

Compare it to whatever you drove before, then decide if you are not getting what you should.

The EPA test takes speeds up to 80mph for bursts. You should be able to get close to their highway estimates traveling at a constant 75mph.

If you need to travel at speeds like 75 miles per hour. try drafting behind a truck. You don't have to get too close. Even 3 or 4 car lengths will keep you in the draft.

On the other hand, if you are passing everyone else on the highway it might do you good to send your right foot to the therapist.

As new a new HCH II owner, I did my research, then drove correctly to maximize FE. I had to change all of my driving habits, and just take it easy. My first tank came out at 50.2mpg. Awesome! The potential is there, but you must work at it and SLOW DOWN

Do not get any closer than this. Far too little gain for the excessive risk.

Someday I might even beat that wascally kenny!

No whammies... no whammies... no whammies...

you simply cannot drive this car as you do a non-hybid and expect to make big fe numbers. read tarabell's article.

Note to Self: Stay out of assist!

But your mpg will be better than whatever you were driving before. That's the approach you should take if you have to drive in that manner.

Maybe I'm just lucky or something but I drive 50miles to work one way, it's not all even ground but it is mostly highway. I use Costco gas, use my A/C and do between 65 and 70 and still get an average of 55mpg, today I got 60. I think if you read the cleanmpg.com article it makes a big deal about consistency and using your gauges, that's what they're there for. You dont have to drive like a grandma, but you do have to adjust your driving habits. I treat my car like a game and I'm always trying to beat my high score, that mentality has really helped me get better.