Speedo, circumference and those &*%$ tire sizes

[Grant]

See question below, but a quick overview of the basics.

The thing that affects speedo error, gearing, and differences between 
tires (which is a huge issue with quattro) is quite simply tire 
circumference.  Circumference is the distance all around the tire, adn 
therefore the distacne it travels in each turn.  Circumference, if we 
all recall geometry, is 2*pi*r, where r=radius and diameter = 2 x 
radius = distance from tippity top to bottom of your total tire + 
wheel.

So what do all those numbers mean?  This is important, since with them 
we can easily calculate diameter, radius and therefore circumfrance.

205 = width of tire in millimeters
/60 = percentage of width that is the sidewall height. In this case the 
sidewall is 205mm x 60% = 123mm
HR15 = (h speed rating, R=radial, 15" = wheel diameter)

So the total circumference of this wheel, mounted on a 15" tire, is:

wheel diameter, 15"+  2 x sidewall height = 2 x 123mm = 246 mm = 9.68"

so total wheel diameter = 15" + 9.68" = 24.68"
The radius is 24.68/2 = 12.34",
The circumference is 2 x 3.14 x 9.68 = 77.51"

For any other combination, you have to do the same math.  I generally 
believe that anything under 2% is OK.
I may even go 3% if I knwo I want a bigger tire and am willing to deal 
with the error.  I did this winter in fact, using a 225/55-16 winter 
tire in place of my 255/40-17 summers.  Pothole protection.

If you want my handy-dandy spreadsheet, in which you punch in your own 
numbers, just email me.  This list does nto allow attachments, I 
believe.

Grant

> From: mboucher70 at hotmail.com
> Date: November 9, 2005 8:03:48 AM EST
> To: quattro at audifans.com
> Subject: Do Lower Profile Tires affect Speedometer Reading
>
>
> I'm looking at a pair of 'performance winter' tires.  The size is 195 
> 55 R15.
>
> My car's original equipment is 205 60 R15.
>
> I've read that for winter you should change the sizing by one each - 
> that is either
>
> 205 60 R15 or
> 195 65 R15 or
> 185 70 R14 if load capacity allows.
>
> I'm a bit confused by this rule.  I understand that the tire profile 
> change is ofset by a rim size change.  So then, wouldn't 205 60 R15 
> and 195 65 R15 give different speedometer readings, since the rim size 
> is the same but the profile is different?
>
> Bottom line question is will moving from 205 60 R15 TO 195 55 R15 work 
> on my car (1990Audi100) or will it change my speedometer readings?
>
> Other question, where is a good place to get inexpensive winter rims 
> (Wal-Mart, etc)?  It uses 6Jx15 rims.  Are these standard rims, or 
> specialized Audi rims?  I've noticed that most Audis use the 5-bolt 
> tires, but mine uses a 4-bolt.
>
> Thanks
>
> Charles
>