Spare tire - 175x55x18

[nene]

Has anyone noticed that the spare tire measurements are 175x55x18? The stock US size tires are 255x40x18.
If you fully inflatethe spare tire, and have to drive on it, doesn't it mean that one wheel is not matching in rolling diameter?

I'm not sure what the correct formula is, but it seems that the rolling diameter would be different between the spare and stock US tire. If that is the case, I guess you really can run different rolling diameters with Quattro, since it's like that from factory.

Any thoughts? Am I just allucinating?

[jinglis]

Can't help with the RS6, I have an S4, but you there are two things you can do about checking wheel size

1. Go to this website

http://www.1010tires.com/TireSizeCalculator.asp (http://)

Very good for your comparison

2. Calculate yourself

Not as good

Here are the forumulas you need

Given a tyre
Width 255mm
Ratio 40
Wheel Diameter 18"

Calculate tyre side wall height (and convert to inches)
(255*40/100)/25.4 = 4.016" (rounded up)

Radius of wheel & tyre

(18"/2)+4.016" = 13.016" in radius or 26.032" in diameter


Circumference of wheel

26.032" * 3.14159 = 81.78"

some people use pi to 4 dp

26.032" * 3.1415 = 81.77"

So distance travelled by one revolution of the wheel is 81.78"

[nene]

Thank you.

[jim kelly]

Learn somethin' everyday at RS6.com.
No spare tyres in any OZ delivered beasts. All have 'run-flat' inflation cannisters fitted.
Thanks also from me for the rollin' dia. calcs site!