I'm old, stupid, and stubborn, and wheel/tire specs confuse the crap out of me.

My present wheels are 19 X 9 and have a 6.7" backspace and 42mm offset

I'm considering another set of 19 X 9, 6.6" backspace and 40mm offset.

Tires remain the same, so on paper, the differences seem negligble.

What does each number represent? Will the new set and lower values be deeper than my originals? In otherwords, which spec causes the tire to be wider within the wheel well?

 

Unless you're real close to rubbing as is, the new wheels won't be in anything's way. That's only 100 thousandths difference (less with the offset).

Offset is the measurement of the mounting surface of the wheel from the centerline of the wheel if you were to cut it in half. Positive offset means the mating surface is outboard of the centerline, negative means further inboard. 0 offset means the two are in line. Backspacing is the distance from the mating surface of the wheel to the inboard edge of the wheel.