I'm quite sure my 14" rims are 3.5 BS I'll do some measuring this weekend.
I hate that everyone talks about and looks at backspace. offset is what is really important, it's measured from the center plane of the wheel.
rim width is bead seat to bead seat, not outside edge to outside edge. generally, the outside edge to outside edge measurement is 1" wider than the wheel width (bead seat to bead seat measurement)
If your 14's are 5.5" rims, the overall width is 6.5", so 0 offset would be 3.25" backspace, and 3.5" backspace would be .25" positive (towards the outside of the wheel) offset.
a 15x7 rim is 8" wide overall. 0 offset would be 4" backspace, so 3.5" backspace is 1/2" negative offset, (towards the inside of the wheel).
Comparing these two wheels, the center of the 15x7 wheel is 3/4" more inboard on the wheel, pushing the outside edge of the tire 3/4" outboard compared to the above 14x5.5" rim for a same width tire.
for a 15x7 wheel to sit in the same place side-to-side as the original 14x5.5" wheel example, you'd need a 15x7 with 1/4" positive offset, which would be 4.25" backspace. that way the center of the wheel is at the same position, so theoretically, a 215/70/14 tire on the 14x5.5" 3.5" backspace rim will be in the same place in the wheelwell as a 215/60/15 on a 15x7" 4.25" backspace wheel (ignoring potential width difference from being mounted on the wider rim)