[bwagner]

What a clickbait title.

This is relatively important information regarding deciding between 18” and 19” wheels and tires that I had not seen posted yet. This is most relevant for someone coming from an E9x (or E82 in my case). Everything below is based on stock wheel and tire sizes.

There is as much tire sidewall on a 19” F3x setup (255/35-19) as there is on a 18” E9x setup (255/35-18). I realized this, and it really changed my perspective. The sidewall height difference between 45-series and 40-series tires is pretty dramatic. I kept thinking about using my E82 winter tires on an F36, but the overall height difference is quite large making for a questionable choice. I made this spreadsheet to find equivalent sized tires. The formula to find overall tire diameter is

Wheel Diameter + 2*(Tire Width * Aspect Ratio / 100 / 25.4).

With this formula, you can calculate the overall diameter (or revs per mile) from a tire size. There is some variation between tire models, but not enough to consider in most cases.



The red highlighted cells are within +/- 1.5% of F3x nominal sizes. I know that is tighter than the recommended 3% variation, but I wanted to find sizes really close to stock so I cut that in half. The stock staggered E9x sizes are highlighted in blue. If you look at the 255/35-18 size (stock E9x rears), the overall diameter is 25.03”, exactly 1” less than the 255/35-19 (stock F3x rears).

I needed to see it graphically. So I drew this up to show the comparison.



The tire sidewall height is ~3.52’ for both.

Now to make this about me! I bent two 18” wheels last year hitting potholes while on the stock run-flat tires, so I was loathe to run 19” wheels at all on a F36. This may change my mind, or at least ease my panic for this winter. I have felt that the run-flat tire construction with its really hard sidewalls was a larger factor is bending the wheels than the firmness of the suspension or even the low profile tires. The run-flat tires have much less sidewall give than standard tires (at the same sizes), especially on radial impacts. The run-flat sidewall has to be more rigid radially (from center to edge) than a regular tire so it can work without air, but this is terrible when hitting potholes, the impact is transmitted from the tread to the wheel without much sidewall flex.

I was planning on buying square 18x9” Apex ARC-8 or EC-7 wheels and use them for DD and mild track duty. My concern is that the resale market for F3x wheels seems to be in 19” or 20” sizes, especially not square 18” setups. I don’t want to spend the extra money or weight on 19” wheels & tires, but if I cannot find a buyer for 18” wheels when I am done with them, it may be cheaper in the long run to spend the money upfront on 19s.

B

[SflBimmer8484]

Keep 19'' tire prices in mind as well. It may add up quick if you will be running the track.