Your speedometor is calibrated by a cable that connects to your transmission, so unless you got a new transmission, don't worry about the speedo
If the dealer put them on your car and somethings wrong, i'm sure you can just take them back. Those tyres (tires where i'm from :P) should fit with your car with no problems. If something feels funky, go to the dealer. I seriously doubt it will eat your gas mileage. It would do that if the tire was taller, not wider. If it downed it at all, it'd be an insignifigant notice. Good luck sir.