Did you mean a 20X9 wheel? Or did you really mean a 20X19
Here's a 20X9 wheel in a +6 offset a 4.5" backspace:
Same truck, with a 20X9 wheel, +18 offset and 4.5" backspacing:
Kinda demonstrates how the more positive the offset, the further in the wheel tuck in.
Check out this site for an explanation on how to measure them both: