F80 stock wheel and tire sizing

[SteveYem]

I apologize if this has been covered ad nauseum and years ago. I’m new to the F80 world and trying to figure something out with regards to the stock wheel and tire sizing for the F80- specifically the 437M and 666M setups.

My understanding is that the 437M wheels are 19x9 et29 front, 19x10 et40 rear, with 255/35/19 front and 275/35/19 rear tires.
Meanwhile, the 666M wheels are 20x9 et29 front, 20x10 et40 rear, with 265/30/20 front and 285/30/20 rear tires.

So, the 19” and 20” wheels are the same widths and offsets.

The rolling diameter of the 255/35/19 front tire is 26.02”, while the 265/30/20 front tire is 26.26”. For reference, a 265/35/19 is 26.30”.

The rolling diameter of the 275/35/19 rear tire is 26.58”, while the 285/30/20 rear tire is 26.73”. For reference, a 285/35/19 is 26.85”.


Due to the relatively taller sidewalls of the 19” tires vs. 20”, I can see where going with 255/35 and 275/35 on the 19” wheels would result in similar sidewall angle (‘stretch’) as the 265/30 and 285/30 tires on the 20” wheels; does that tire stretch serve a functional purpose for handling / cornering, as in, is the tire sidewall less prone to ‘rolling over’ when it is stretched like that?
In any case, it comes at the expense of grip due to the relatively thinner tread widths used on the 19” setup.

Finally, it looks to me that if they had gone with 265/35 front and 285/35 rear tires on the 19” setup, the resulting rolling diameters would have been closer to the 265/30 and 285/30 tires on the 20” setup while matching the tread widths.
At factory stock ride height and without wheel spacers, I don’t believe the 265/35/19 and 285/35/19 setup would have resulted in any rubbing issues.

So what am I missing? Why did BMW go with 255/35 and 275/35 instead of 265/35 and 285/35 for the 19” option?

[fwrigley]

I can't comment on why BMW chose what they did, but the 265/35/19 is a front tire they offer on some models (CS & GTS for example). No rubbing issues at all with that tire. I believe some people report inner fender rubbing with a 275/35/19 in the front so 265/35/19 is likely the largest they would go on a factory setup.

Relative to your question on the stretch of a tire and its impact on feel/grip, TireRack did a comparison recently that talks to this. Summary is that wider wheel widths generated the lowest lap times regardless of whether a 225 or 245 width tire was utilized. They did the tests in both 2022 & 2023 so I linked to both articles below.

https://www.tirerack.com/tires/tests...y.jsp?ttid=296
https://www.tirerack.com/tires/tests...y.jsp?ttid=313

[MineralWhiteF80]

Originally Posted by SteveYem

So what am I missing? Why did BMW go with 255/35 and 275/35 instead of 265/35 and 285/35 for the 19” option?

I think it's easy to overthink this topic, and no one will ever really know (maybe not even BMW), but it's very likely the answer consists of these items (plus many others), and this is purely my guess, so don't shoot me.

1. The generic Width/Sidewall/Diameter numbers printed on the tires do not tell the whole story of a tire's actual dimensions. Some tires are actually wider in the same nominal size as other tire models.

2. Development of the standard and competition models were designed before the first 2015 F8x chassis was released, even though the Comp model didn't debut until 2018. To make the competition model stand out as more sporty, they likely went up a width in generical tire sizing so they could say "Comp package has WIDER tires for more grip!" or, they purposely made the standard M437 and M513 wheels have narrower tires so they could put the 265/285 tire on the most sporty model at the time. I wont mention the fact the M666 is substantially heavier and detrimental to performance LOL.

3. Tire stretch is important for performance, and BMW likes to stretch their tires. However, you can get away with less stretch on a lower-profile tire and still have acceptable sidewall support. So it is likely BMW felt it was OK getting 0.3" wider tread width from the +1 sizing because the thinner 20" sidewall could still offer acceptable support.

4. BMW might not have had access to those size tires since there isn't a 285/35/19 size available in a Michelin Pilot Super Sport and the PS4S model was not available back then. The other factory M437 tire option was the Continental ContiSportContact 5P (which I HATED) and was not available in either 265/35/19 or 285/35/19 tire sizes. So maybe the M437 didnt even have the possibility of a factory 265/285 tire back in 2015+.

I could go on, but I wont.

The tire overall diameter thing really is not the issue here, but the largest tire diameter you can go up front without major issues is 26.3" and issues arise only when you start spacing the wheels out so they are flush with the fenders and lowered suspensions.

I will say that a 265/35 and 285/35 tire size fits fine on the M437, but the performance gains (especially on the street) are negligible at best, aside from a more meaty, bulgy appearance which I think looks better. I've done several track days with both 255/275 and 265/285 sizes on M437 wheels, and my clean average lap times were indistinguishable from each other. Couldnt tell a difference whatsoever and I consider myself a solid Intermediate driver that does some Advanced days depending on the track day organization.

[alex2364]

Only BMW would know why they initially went with 255/35 and 275/35 instead of 265/35 and 285/35. My guess is that with further R&D of the Competition and CS models, they realized 265/285 were better suited for the car. 255/35/19 and 275/35/19 was the first base iteration, second iteration they had 265/30/20 and 285/30/20 for Competition, third iteration had 265/35/19 and 285/30/20 for CS. They also offer M Performance 763M wheel package for all F80/F82 in 265/35/19 and 285/30/20 like the CS. So I'm guessing from all this plus the fact that G80 also has 19/20 in the front and back, 265/35/19 and 285/30/20 is the best optimal combo.