35 vs 40 sidewall for 18's

[cptclutch]

I'm downsizing to 18's and was originally planning on running 40 sidewalls to maintain the stock overall diameter but I'm no longer prioritizing comfort and have been working on making the car as responsive as possible and am now worrying that the 40 sidewall might be counterproductive. Should I just stick with 35's? The speedometer being slightly off isn't a huge deal to me.

[MineralWhiteF80]

Speaking in generalities (and purely opinion), there is something I like to call the 'golden rule' regarding the ratio of sidewall height to wheel diameter. Get this ratio wrong, and your wheels and tires can look stupid . Wheels and tires that don't fit well together or that provide a bad stance can ruin any car.

You can't only look at the sidewall percentage numbers for appearance and/or ride quality (e.g., 35 sidewalls vs. 40 sidewalls). You need to look at tire width AND sidewall percentage together. With that said, you can have a larger sidewall percentage number (i.e. 40) with a narrower tire width or a much wider tire and smaller sidewall number and still fall within the goldilocks zone of the 'golden rule.'

For the F80/F82 chassis, the sidewall thickness that most people find look the best is somewhere in the 4.0 to 4.2" sidewall thickness. Anything skinnier you get into the "rubber band" territory, and anything bigger you get into the "Sport Utility Vehicle" category. In person, 18" wheels typically look small on the F8x chassis unless you get the tire sidewall thickness correct to fill out the wheel arches more. Pictures of F8x on M3post usually make an 18" wheel look larger than they do in real life (especially if the car is lowered a lot).

If you are looking for responsiveness, the treadwear and performance category of the tire plays a big part. You can take a 40 series 200 treadwear performance tire that generally has a stiffer sidewall, and it will be more responsive and direct than a 35 series 300 treadwear street tire with a softer, more pliable sidewall.

Have you chosen your wheel widths/offsets yet? Depending on what width your wheels will be will dictate what the tire width should be for proper sidewall support, and your tire width should help you calculate your ideal sidewall height that is appropriate to fall into the golden rule of sidewall-to-tire ratio.

[N1rve]

Originally Posted by cptclutch

I'm downsizing to 18's and was originally planning on running 40 sidewalls to maintain the stock overall diameter but I'm no longer prioritizing comfort and have been working on making the car as responsive as possible and am now worrying that the 40 sidewall might be counterproductive. Should I just stick with 35's? The speedometer being slightly off isn't a huge deal to me.

If you want a better responsive handling experience, then you want bigger rims (19s) that are roughly the same weight as the 18s such as forged rims. You want less tire flex which means a shorter sidewall by ratio or filled with rim material or XL tires which result in stiffer sidewalls. Another way is to increase tire pressure but not by too much or else you will get over inflated tire wear.

35's will leave you with a bigger wheel gap if that matters to you lol.