Stock is
Size 18x8
Offset +19.05
This means the offset is closer to the outer side of the wheel, (the face of the rim away from the caliper)
It is 19.05mm away from the centerline of the rim. This is called positive offset.
so if you wanted to say, go 2 inches wider, and be able to keep the inside of the tire away from rubbing, and you want all that extra width towards the OUTER side of the truck, you’d take your added width, and convert it to metric, 2 inches to metric is 50.8mm,
Subtract that from your positive offset of 19.05, bringing your desired offset to -31.75.
this should maintain the same distance as factory, putting your total offset OUTSIDE of the truck.
however, I’d look into your fender well and set what looks like it could interfere if you went with a larger tire. But you should be pretty safe with that offset, because the inside of the rim will be in the same place as factory, and the outside will be your desired 2 inches of protrusion.
link explains it pretty well.
TIRERACK (understanding wheel offset)
