Wheel size/offset questions

[Sirrith]

Hello all

I'm planning to change my stock 17" rims to some 18" Enkei T6S soon (avoiding 19" because of ride quality, risk of pothole damage and price), and I'm having a bit of a hard time with the offset/width calculations as I am seriously mathematically challenged.

As far as I can work out, my 2009 23i has a front wheel size of 17x8 29ET and rears of 17x8.5 40ET.

My first bunch of questions are, do I want/need to get wider wheels as well as larger? If so, should I be getting 18x8.5 fronts and 18x9.5 rears, or 18x8.5 and 18x9 to keep with the stock "ratio"? Does it make much difference besides aesthetics?

Next, what offset range would work? I don't want to use spacers. Am I correct in thinking that if I stick to the stock offsets of 29 and 40, I should be good to go even with wider rims? Or is there a different offset recommended that would look/perform better because of the increased width of the rims? I'm seeing so many different combinations in my research that it's making my head spin.

Thank you!

[Player 1]

The sizes of my wheels are as follows;
Front - 19x8.5 ET38
Rear - 19x9.5 ET40

Those offsets are perfect, almost identical at the front and wider at the rear to fill the arches.
I assume the same width wheels in 18 inches wouldn't affect offset?

[Sirrith]

The sizes of my wheels are as follows;
Front - 19x8.5 ET38
Rear - 19x9.5 ET40

Those offsets are perfect, almost identical at the front and wider at the rear to fill the arches.
I assume the same width wheels in 18 inches wouldn't affect offset?

Thanks, ET38 on the fronts and you get no rubbing or other issues??

[brillomaster]

have a look at willtheyfit.com, good visualisation tool on the effects of widths, offsets and tyre sizes.

generally, if you go for wider wheels, you'll also need to decrease the offset. As a wheel that is 1" wider with the same offset will stick out an extra 0.5", but also stick in a further 0.5", which may begin to foul suspension components.

to be honest, an 8.5" rear rim will fit a 255 width tyre, which is more than enough for the power a 23i makes. the only reason to go wider would be aesthetics.

[Player 1]

The sizes of my wheels are as follows;
Front - 19x8.5 ET38
Rear - 19x9.5 ET40

Those offsets are perfect, almost identical at the front and wider at the rear to fill the arches.
I assume the same width wheels in 18 inches wouldn't affect offset?

Thanks, ET38 on the fronts and you get no rubbing or other issues??

Front's absolutely fine, that offset means the extra width is almost entirely on the inner face of the wheel where there's plenty of space.