The strange world of tire sizes

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garsh

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Tesla's Model 3 Track Package comes Zero G wheels with 245/35ZR20 XL Michelin Pilot Sport Cup 2 tires.
Here are the specs for these tires:
1587664836021.png


The OEM tires for a Model 3 Performance are 235/35ZR20 Michelin Pilot 4S tires.
These are also the tires that are mounted to the Zero G wheels when you get them as a referral prize.
Here are the specs for these tires:
1587664961550.png


The tread width is only 0.1" different. But the Zero G wheels are 9" wide, while the OEM wheels are 8.5" wide. So I was wondering why Tesla didn't put a wider tire on.

So I looked up the stats on the 245/35ZR20 Michelin Pilot 4S tires. This should be *slightly* wider than the OEM tires, right?
1587665205706.png


Uh... why is the tread width for the 245 version of the tire less than that of the 235 version?
Thoughts @Mad Hungarian ?
Regardless, the larger section width would probably make the tire appear less "stretched" when mounted on a 9"-wide wheel.
 

Sugi

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Interesting find. This made me wonder on the Zero-G wheel running the 245/35/20. There is a gentleman "Isaac" on MPP's website that is running 18x9.5 +35 wheels with a 275 tire?!? The Zero-G "I believe" is a 20x9 +34 and is only running a 245??? Math hurts my head.
 

Mad Hungarian

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Tesla's Model 3 Track Package comes Zero G wheels with 245/35ZR20 XL Michelin Pilot Sport Cup 2 tires.
Here are the specs for these tires:
View attachment 33571

The OEM tires for a Model 3 Performance are 235/35ZR20 Michelin Pilot 4S tires.
These are also the tires that are mounted to the Zero G wheels when you get them as a referral prize.
Here are the specs for these tires:
View attachment 33572

The tread width is only 0.1" different. But the Zero G wheels are 9" wide, while the OEM wheels are 8.5" wide. So I was wondering why Tesla didn't put a wider tire on.

So I looked up the stats on the 245/35ZR20 Michelin Pilot 4S tires. This should be *slightly* wider than the OEM tires, right?
View attachment 33573

Uh... why is the tread width for the 245 version of the tire less than that of the 235 version?
Thoughts @Mad Hungarian ?
Regardless, the larger section width would probably make the tire appear less "stretched" when mounted on a 9"-wide wheel.
Oh yeah, I see this all the time, check this post.
 
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garsh

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Oh yeah, I see this all the time, see this post.
Even though I know enough to expect a wide variation in tread width across tires of the same size (due in no small part to your shared wisdom), I still assumed that a wider tire OF THE SAME MAKE AND MODEL would end up having a wider tread, not a narrower tread width.

Color me shocked. o_O
 
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Mad Hungarian

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Even though I know enough to expect a wide variation in tread width across tires of the same size (due in no small part to your shared wisdom), I still assumed that a wider tire OF THE SAME MAKE AND MODEL would end up having a wider tread, not a narrower tread width.

Color me shocked. o_O
Oh believe me it gets even crazier... we once saw an example from Pirelli where Audi had them build a spec tire for the A5 that had a different O.D. from any other tire that size. We had their engineering check the data sheet because we were sure it was a mistake, it wasn't. That was some Twilight Zone stuff.
 

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So, @Mad Hungarian, what is the surest way for us tire idiots to protect our OEM 18" rims when purchasing new tires? What specs should we be looking for? Obviously we want to minimize range loss.
 

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So, @Mad Hungarian, what is the surest way for us tire idiots to protect our OEM 18" rims when purchasing new tires? What specs should we be looking for? Obviously we want to minimize range loss.
Well, the sad reality is that there is no way to increase tire width without losing range. But having said that, if you stay with a low rolling resistance model and go up just one size to 245/45R18 it isn't going to be a huge difference, perhaps 2% or 3%.
An alternative would be to stay with OE size and add some rim edge protectors, like these lovely units from Evannex.
 

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Even though I know enough to expect a wide variation in tread width across tires of the same size (due in no small part to your shared wisdom), I still assumed that a wider tire OF THE SAME MAKE AND MODEL would end up having a wider tread, not a narrower tread width.

Color me shocked. o_O
Tires are sized in section width, NOT tread contact width. Actual tread contact patch size depends on rim size, inflation pressure, applied weight (load), etc. And don't forget to factor in side wall rigidity.
 

Mad Hungarian

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Tires are sized in section width, NOT tread contact width. Actual tread contact patch size depends on rim size, inflation pressure, applied weight (load), etc. And don't forget to factor in side wall rigidity.
Correct, here's my fuller post on the subject from the Replacement Tire thread from a while back...
Sure thing, let's actually take a quick dive into how Euro-metric tire width dimensions really work, like most things in my life it's a bit weirder than you might think...

A common misconception when talking about tires that the width number shown in the size (in this case 235 mm) is a measurement of how wide the tread is. This dimension is actually the Section Width, which in is the widest point of the tire as measured from sidewall-to-sidewall. See the diagram I just borrowed from Continental below.
You may notice there is also an additional dimension called Overall width which is just a hair larger, that would be the Section Width plus the added height of any protruding lettering on the sidewall, but this isn't commonly used for fitment purposes.
1587754095286.png

As we can see, the Tread Width is somewhat narrower than the Section Width and can also vary quite a bit for a given tire size due to differences in the curvature of the shoulder area, where the tread edge meets the sidewall. A tire with a very "square" shoulder will tend to have a wider tread width than one with a much more rounded shape. This is open for the tire company to determine.

Although the measured Section Width dimension IS a defined standard governed by tire manufacturing oversight organizations (in this case the ETRTO), it's interesting to note that the Section Width is not a "fixed" dimension per se, it will vary with the width of the rim you install it on. This is why in all the detailed tire specifications you will see a listing for the Permissible Rim Width range, which shown the narrowest and widest rim widths that the tire may be legally installed on, and also the Measured Rim Width which indicates which exact width of rim the tire was installed and inflated on to obtain the rated Section Width. If that wasn't enough, the rated Section Width very often isn't exactly the dimension we expect either. Here's an excerpt from our tire database that shows the full ETRTO specs for a 235/40R19:

1587754149313.png


Notice something interesting the 14th column, Design - Section Width on measured rim? It says 241, doesn't it? That's not a mistake, this how wide the ETRTO says a 235/40R19 is supposed to be when installed on the specified 8.5" wide rim. So even under the best of circumstances a 235 is NOT really a 235. And for an idea of how much it widens or narrows depending on the width of rim chosen, have a look at the Section Width vs. Rim Width columns at the end.

As to what the pros/cons are of a wider Tread Width vs a narrower one, that's just one of a huge number of variables that go into tire design and it's kind of outside the scope of this thread to get into it (in fact the so is the whole Section-Tread width discussion, but I figured we could cheat a little & kill an oft-asked one). What matters is that the whole package delivers on what it was designed to do.
 

garsh

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Tires are sized in section width, NOT tread contact width. Actual tread contact patch size depends on rim size, inflation pressure, applied weight (load), etc. And don't forget to factor in side wall rigidity.
All that is true. But I still found it strange that the
  • Same make/model tire
  • Mounted to the same width of rim
  • Inflated to the same pressure
  • BUT, having a larger section width.
... ends up having a smaller tread width.
It just blows my mind. :)
 

Klaus-rf

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All that is true. But I still found it strange that the
  • Same make/model tire
  • Mounted to the same width of rim
  • Inflated to the same pressure
  • BUT, having a larger section width.
... ends up having a smaller tread width.
It just blows my mind. :)
That makes sense. Except for extremely small sizes in section size, a different rim size is needed for a wider tire. Each tire size has a "recommended" rim width size (one size) and possibly also a "acceptable rim width" range (with something like 8.0-10.0").

But it's been decades since I worked for a tire mfgr so my memory may be a but faded.