Good question. Where's @K-A when you need him? In pics it looks like the Macan on 20s with Steel springs is higher than the 19s. I actually was just looking through a bunch of pics and noticed this. But it was prob the angle of the photos. 12mm is about a half inch so not sure if that would even be noticeable in pics.

 

I thought about it a little more and here is my answer:

1- The car will be taller by 7mm / 2 = 3.5mm

2- The wheel gap will be smaller by 7mm / 2 = 3.5mm

This happens probably because the center of the wheel is higher by 3.5mm in the 20s compared to 19s.

Am I stupid?

 

Silly question!
Based on my calculation the tire + wheel diameter for 19s and 20s are like this:

19s: (250*0.5*2)mm + 19inch = 732.6mm
20s: (295*0.4*2)mm + 20inch = 744mm

So, on a Macan S with PASM and no air suspension, does this mean the car will be higher by 12mm? or with 20s the wheel-body gap will be smaller?

19s is 255, so it is 737.6... and the car will be higher for one half of the difference!

 

Yeah, changed 250 to 255, I had it wrong. Thanks for catching that.

I reached the same conclusion, half of the diff. So the car will be 3.5mm higher which is almost nothing.

 

Suggest just looking at the specs for respective tire sizes (Tire Size Comparison). There are minor variations in OD of different brands for a given tire size. Either way, the difference is virtually insignificant visually. And yes, the difference is divided by 2 for impact on vehicle height impact.

 

This discussion makes no sense.

First, the 255 and 295 numbers represent the tire's width and have nothing to do with the circumference, diameter and wheel gap.

If you think that the 20" wheel/tire has a smaller gap, that means the overall diameter circumference is bigger and therefore a Macan with 20s goes faster than a Macan with 19s for the same horsepower????

I gave up after 10 minutes trying to find the same tire that has both sizes in it's overall production specs, but see this representative chart.

Click the Specs tab. The only numbers that matter to this discussion are the far right columns: Overall Diameter and Revs Per Mile. They vary slightly by tire, but the combined circumference effect of 19, 20 or 21 is de minimis.

http://www.tirerack.com/tires/tires.jsp?tireMake=Pirelli&tireModel=P+Zero+Rosso&partnum=845WR90AR&GCID=C13674x012-tire&KEYWORD=tires.jsp_Pirelli_P_Zero_Rosso_Tire&code=yes&srccode=cii_13736960&cpncode=43-7172651-2



Bonus Question for Extra Credit:

If a car has three 20" wheel/tire combinations on it and you mount the 19" collapsible spare, how many inches off the ground is the 19" tire?

A) 1 inch
B) 1/2 inch
3) Zero inches

 

This discussion makes no sense.

First, the 255 and 295 numbers represent the tire's width and have nothing to do with the circumference, diameter and wheel gap.

Actually, the width does have something to do with the circumference and tire's outside diameter, since the wall height is a function of the width times the aspect ratio (235, 255, 295 etc). So the wider the tire, the higher the sidewall for any given aspect ratio, and therefore the larger the OD.

 

This discussion makes no sense.

First, the 255 and 295 numbers represent the tire's width and have nothing to do with the circumference, diameter and wheel gap.

QUOTE]

Easiest of all is to look at the tables on tire rack and other sites and compare the revs per mile figures or diameter figures. But, if you want to figure it out on your own, you can calculate the tire diameter yourself, and in that case the tire width has everything to do with it. see tire rack article http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=7

Basically, the total diameter will be the rim diameter plus two times the tire section height. To calculate the tire section height, you multiply the section width (235) by the aspect ratio (.55) to arrive at a section height of 129.25 mm. Divide by 25.4 to convert to inches yields 5.089. So, 19+5.089+5.089 = 29.177, which rounds up to the 29.2 inches shown in the table.

 

From what I read on that link above, Wheel Width plays a role as well. And probably the tire inflation.

 

No, the width is independent. It represents the width of the contact patch.

For example, a 19" Pirelli P Zero Rosso tire with a width of 255 can be purchased with aspect ratios of 35, 45 and 50. Each has a different Overall Diameter and Revs Per Mile.

 

I have been using tall tires to reduce wheel gap for years. This allows for a more moderate spring drop with the rest of the gap being taken up by the tire height.

My snows are even taller for the macan. 275x45-20 will be 756mm tall. Half increases ground ride height and the other half decreases the wheel gap. A 235x60-18 snow is 739mm tall, so mounting the ones I have is raising the ground clearance by 8.5mm and reducing the wheel gap by 8.5mm.

The tire height is the real number one uses for matching fronts and rears on an awd vehicle. Staggered works as long as the heights are close.

The big 21" 295s are actually only 739 mm tall too.

http://www.rimsntires.com/specspro.jsp

 

Looks like everyone's right. Here's a paragraph from tirerack.com link :

"Actual Tire Section Width is Dependent on Wheel Width
All tire sizes are assigned specific rim widths upon which they are measured (measuring rim), as well as can often be mounted on slightly narrower or wider wheels (rim width range). Therefore, it is important to note that actual tire section width will depend on the wheel width the tire is mounted on. The rule of thumb is that tire section width changes by 0.2" for every 0.5" change in rim width, being reduced if mounted on narrower then measuring wheel and increased when mounted on wider wheels."

And I believe @keninblaine is also right. If the aspect ratio is fixed, say 55%, then wider tires will make the diameter larger. And I see where TTCarrera is saying too. Porsche specifies certain overall diameter and it results in different size combination of width/wheel diameter/aspect ratio.

At any rate, different tire/wheel sizes on Macan have negligible impact on wheel gap. If it did, we'd probably have a problem with speedometers.

 

And I believe @keninblaine is also right. If the aspect ratio is fixed, say 55%, then wider tires will make the diameter larger.

At any rate, different tire/wheel sizes on Macan have negligible impact on wheel gap. If it did, we'd probably have a problem with speedometers.

Sorry, but once again you don't start with a fixed aspect ratio. The aspect ratio is the result.

Starting with a fixed aspect ratio would be like saying in March that everyone on the Red Sox would bat .305 for the season.

I grew up in the tire business. My uncle was the GM of a major tire manufacturer's production plant and ended up buying that facility and producing product for several brands. I worked there when I was in college and for a few years after. We ended up selling to one of the other major brands.

Width, compared to diameter, results in the ratio.

 

20's are for girls. 21"-ers the way to go!>

 

I get the math, it's simple. The question is, of the three, which two do manufacturers set: diameter, AR, width. The third obviously follows. If you surf tire rack etc, you'll notice that they always spec even (to the 5mm and 5%) numbers for width and AR, which leads me to believe that diameter is the result of the calculation. They may start with a goal for diameter, but in the end it comes out to 'off' numbers like 29.74 ". You can spec a diameter and then 'back into' an AR, then figure out your new diameter when you round to 5mm. The fact that diameters are all weird, as I mentioned, leads me to believe that they spec width and AR, and the diameter is what it is, no matter if it was the end target or not.

 

A= width, B= ratio, C= height of one sidewall, a x b = c, c/b=a, c/a=b, It's all the same.

Tire height or diameter just adds the rim and both sidewalls. X Pi is circumference.

We know that the speedometer is 3km/hr fast. Going taller makes the speedometer closer to actual, and reduces tire gap.