RE: Installing larger rims/tires...

["Squier, Craig" <craig_squier@arvin.com>]

Huh?
Since Diameter times Pi is Circumference, you will get the same percentage
error whether you work with diameter OR circumference...

Remember:
%Error = (Stock Diameter-New Diameter)/Stock Diameter x 100

Note that postive numbers mean that the speedo read this percentage higher
than actual, where a negative percentage means the speedo will read this
percentage lower than actual.

For circumference since everythin is times Pi, the Pi divides out as 1. no
difference, just easier to do on diameter...

Example: Stock Jetta GLX

Tire Size: 205/50-15
Diameter (in inches) = 23.071"

Plus One : 205/45-16
Diameter = 23.264

Error = (23.071-23.264)/23.071 x100 = -0.84%

A plus one tire will result in your speedo reading 99mph when you are doing
100mph.

There is one additional sticky factor...
Now that the speedos are electronic there is a signal response curve
associated with the speedo. At extremely low and extremely high speeds,
there will be a variance in the speedo. So far it looks like the speedo
reads slightly higher than actual road speed. When I am doing 60mph (by
speedo) I am actually doing about 57 mph road speed (by time and mile
markers).

I think this error increases to as much as 10mph at top speed...

Hope this helps all..
-Cab

> ----------
> From: 	CJMarshall@aol.com[SMTP:CJMarshall@aol.com]
> Reply To: 	jettaglx@igtc.com
> Sent: 	Tuesday, January 26, 1999 10:16 AM
> To: 	jettaglx@igtc.com
> Subject: 	Re: Installing larger rims/tires...
> 
> I agree with your logic, but disagree with your actual calculation.  You
> can't
> go by the diameter to calculate speedo variance.  You have to use the
> circumference of the tire and see how much that has changed to be able to
> determine %+/- from stock.  An interesting thought that I'll work on.
> 
> Chuckem
> 
> 
> 
> In a message dated 1/25/99 7:07:09 PM Eastern Standard Time,
> sundie@lunaticfringe.org writes:
> 
> > There's a certain amount of "work around" you can do in the case of
> >  getting larger rims.  If you lower the profile, you can (usually) find
> a
> >  profile/rim combination that's pretty close to your standard.
> >  
> >  For instance, before i went and changed to my winter tires, i checked a
> >  few things out, did a few calculations...
> >  
> >  with the winter tires on (195/55/r15), the profile of the tire/rim is
> >  102% of standard (205/50R15), and with the summer tires on
> (215/45/r15),
> >  the profile of the tire/rim is 98% of standard.  I could have gone with
> >  the standard sizing, but i wanted narrower tires for winter, and wider
> >  for summer...
> >  
> >  What this means, in working terms, is that with the winters on, and my
> >  speedo says 100, i'm actually going 102, and with the summers on, 98.
> >  
> >  I figured this was pretty safe range.
> >  
> >  You can do the same thing, if you go to larger rims.  Just look around
> >  for tires that have a lower profile, do some simple math, and there you
> >  have it.
> >  
> >  I WOULD suggest doing all of this BEFORE you buy the tire, though.
> >  
> >  Anyone care to take a guess on how much 2%+/- difference from stock
> will
> >  make your car act?  I'm noticing pretty much nothing...
> >  
> >  Sundie....
> >  
>