The tires are 38"x13.5"x18 Toyo MTR and the wheels are 18x9 with a -12mm offset (~4.53 backspace). Very slight rub on the frame rail at full lock, not any more than stock or even the 37s I had. I usually offload this thing pretty aggressively and have never had an issue at full compression of the suspension front or rear. I have 4.56 gears and do not experience any sluggishness or notice the drivetrain working any harder. The truck still has plenty of power. In fact I towed 6,500 lbs of flooring and trailer this morning. However, If I knew 8 or 9 years ago when I re-geared to 4.56 that I was going to eventually end up with 38s I would have went with 4.88 gears. But I have not wanted to change them because quite honestly its not that that noticeable over the 37s I had. As far as fuel mileage, thats a tough one, I really don't check anymore the truck has over 250,000 miles and gets me from A to B without issues. It has a custom performance tune, long tube headers, high flow cats, and a flow master exhaust, and I love the sound of the engine too much to worry about fuel mileage. I would probably say the mileage has suffered slightly because they are a heavier tire and more diameter to rotate. I drive about 40 to 50 miles most days and its been years since I reset the MPG on the DIC, but the truck stays at 11 mpg on the DIC.
I would never go back to 37s I think the proportions of the 38s on the H2 just look and feel right and the leveling kit is perfect for me because I did not want to go with a full lift kit. The truck is more than capable as its currently setup.