Hope someone has the answer to this one. My head explodes when I try to calculate it.

This is for my auto-x/track days applications only.

I have a lowered 06 MCS with 15x7 Kosei K1-TS wheels (38 mm offset). Tires are 225/45-15 Hoosiers - A6 or R6, depending on the event.

Front camber -2.3 to -2.4. Rear -1.3 to -1.4.

The Hoosiers rub slightly on the inside. Using spacers from 3 mm to 5 mm cures the problem. I typically use 5 mm because of the quality of the available spacer.

So my effective ET is 33. But I know it could be 35, from experience with the 3 mm spacers.

Therefore, it could be said that any offset from 33 - 35 is known to work on my car with the 15x7 wheels.

I'd really like to get 15x8 wheels for these events.

Assuming the exact same tires - Hoosier 225/45-15s - what is the offset I need for 15x8 wheels?

 

What part of the tire is rubbing? If you go 1" wider you will be adding 12.7mm to each side of the wheel. That will stretch the shoulders of your tires by that much, but will not necessarily increase the thread width at all. that depends on the width of the tire in comparison to the wheel it is currently on. If you are not rubbing now with effectively a 35 offset, you may not rub with a 8" wheel on that offset. It really depends how close the rim is to the obstruction.

 

Inside edge. But only at 38 mm offset. Not with 35-33.

 

If the inside edge of the tread was rubbing with 38 offset and 15x7, then would
go with a 33-35 mm offset with the 15x8, as the tread should be about in
the same place as with the 15x7s. If you then find that the inner sidewall
or rim contacts the suspension, then adding the 5 mm spacer (which you
already have) should do the trick. Of course, it's possible that could cause
the outer edge of the tread to rub.

If the rim or sidewall near the rim was rubbing at 38, then you'd need up to
13 mm less offset on the 15x8s compared to the 15x7s.

 

Thanks!!!

You may be right, but I have to admit that I'm doubting it.

What about the fact that the 15 x 8 wheel is 1/2" closer to the inside edge than a 15 x 7?

 

Sorry, I also thought about it and edited my response while you were posting this.

If the rim or sidewall near the rim was rubbing at 38, then you'd need up to
13 mm less offset on the 15x8s compared to the 15x7s.

 

That's what I thought initially, but the unknown factor is that the tire is the same section width, only a bit flatter in aspect ratio, so the question is, do I really need the full 13 mm?

Now you see why my head hurts thinking about it...

 

I use 949racing 6UL rims on my lowered (PSS9) MCS.

15x8 with +36mm offset and 225/45-15 tires.

No rubbing in the fronts using stock sized brakes.

Rubs lower rear trailing arms in the back. I use a 3mm spacer. But 5mm spacer is OK with long enough bolts or studs. Spacers only needed for the rears. Doesn't rub my rear swaybar but it could if it were more square.

To fit
15x8 +36 to +40mm offset in the fronts
15x8 +30 to +33 offset in the rears or +36mm offset with 3 to 5 mm spacer.

The bigger the tire the more risk of rubbing with any lowered suspension.
225/45-15 is very short while 225/50-15 is taller but still shorter than stock height. Clearance in the wheel arch inner plastic liner is not a problem with 225/45-15.

 

here's a good tool for determining position when changing offset - click

 

Thanks PGT.

I was there before I posted this.

Didn't get the answers to what happens with the same tires and wider wheels.

Could be well be that I'm missing something obvious.

 

sometimes you won't know anything until you try. doing the math....crunching numbers...."if the glove doesn't fit, you must acquit' or something

 

Minhune says he is using 949 Racing UL 15x8s with the same tires and similar suspension.

So I'll follow his lead and not do any more pioneer work...

Thanks a lot for all the input!

 

Yes, but you'll need some rear spacers, maybe as much as 5mm.
The tires clear but the inner rim edges could narrowly hit the rear trailing arm.

Yours is unusual that you get some rubbing with the 15x7 rims, that should have fit OK with no spacer but there can be some small individual differences.

I think the spacer should be OK if you use a 15x8 rim with +36mm offset.

 

Thanks again!

As I mentioned, I already have the spacers and expect to need them.