Forgive me, but what do you expect? If you put on oversized tires, it is naturally going to make the engine and the transmission work harder. My suggestion, get a chip or a programmer to give yourself a little more muscle to make up for those tires. An exhuast system wouldn't hurt either, every little bit helps. Besides, your truck is nowhere near broken in. That and the overhead console is almost always off by a couple points when it comes to the gas mileage. It shows the average miles per gallon not what you're actually getting. Calculate it yourself, you might a difference and then again you might not. Don't always rely on the console. I hope this helps in one way or another.