How offset & backspacing work
Offset (ET) is the distance from the wheel's centreline to its mounting face. A higher ET tucks the wheel inward; a lower or negative ET pushes it out (poke). Backspacing is the distance from the mounting face to the inner lip:
backspacing = (width × 25.4 ÷ 2) + ET
Going wider or dropping the offset moves the outer lip out (more poke / fender risk) and the inner lip in (closer to struts, brakes and arms). This tool shows both deltas so you can judge clearance before you buy.
FAQ
Is +35 or +45 more aggressive?
+35 sits ~10 mm further out than +45 (lower number = more poke), all else equal. Width changes shift both edges too — this calc combines both.
What about spacers?
A spacer effectively lowers offset by its thickness. A 15 mm spacer on an ET45 wheel behaves like ET30 — add it to the outer-edge poke.