[m340i.kai]

This might sound stupid but i feel like the math is right. If i have 255 rear and i add a 15mm spacers, then my rear width would be 270mm?So it would look the same as someone with 270mm width tires. is that how it works?

[G22b58]

No. If you just consider the outside of the tire (with original ET), a 270 mm wide tire expands both inwards and outwards so to have the outside at 255/2+15 you would need a 255+15*2 = 285 mm wide tire (and theoretically 30mm wider rim).

[m340i.kai]

Originally Posted by G22b58

No. If you just consider the outside of the tire (with original ET), a 270 mm wide tire expands both inwards and outwards so to have the outside at 255/2+15 you would need a 255+15*2 = 285 mm wide tire (and theoretically 30mm wider rim).

So with your math, 285mm tires are equal to 255+15mm spacers?
Im asking this bc im considering 792m wheels but they come in 225 square. And Im wondering how it would look if i have 225mm rear plus my 15mm spacers.

[G22b58]

Originally Posted by m340i.kai

So with your math, 285mm tires are equal to 255+15mm spacers?
Im asking this bc im considering 792m wheels but they come in 225 square. And Im wondering how it would look if i have 225mm rear plus my 15mm spacers.

No, not equal!
I answered what I thought was your question, if a 275 tire at unmodified ET would have the outside of the tire as far out as 255 + spacer 15mm. The answer is that in that case you need 15mm at each side, so a 285mm tire would have the OUTSIDE at the same position.
See other threads for guidance how to choose ET with different wheels and tires.