I have asked the dealer (Van Buick Scottsdale, AZ) why is the car getting poor gas mileage. They checked the cars engine and told me that everything is set to factory settings. I can't see how that can be, my tail pipes are black!. I use 91pr gas and I have hydrogen in my tires, (35 lbs). I use a high grade oil and nothing improves my gas mileage. The dealer said after 5000 miles I would get better mileage, then 10,000 miles but still no change. I called GMC they said there was nothing they could do if the engine was set to the factory settings. The sticker said 17 city 24 highway, not true!