I just dropped a 5.3L engine into my Silverado and I'm shocked at the gas mileage I'm getting now. I get like 17-18 mpg while going 80. Is that normal? I have an intake, exhaust and bully dog programmer. My 4.8L that was originally in the truck was getting like 25 so I don't understand why it would be that big of a drop. I was just wondering if that was the normal mileage for you guys out there with the 5.3. Please help! btw the engine was out of an 04 Silverado. and I have 3.23 gears in the rear end.