Wheels/Tire Combo ?

[Enzeder]

Why bother trying to make the new wheel/tire combo exactly match the stock 
rubber?
1. Isn't the speedo error going to change by a few percent as the tire 
wears?  Roughly 2% if worn to the wear bars by my calculations.
2. How many cars come from the factory with 100% accurate speedos (police 
package cars as a possible exception)? Something about engineering 
tolerances springs to mind.
3. What effect does changing tire pressures have on the rolling radius?
4. How do you maintain and EXACT speed for 5 miles?  Besides the fact that 
the cruise control has long ago died on my 5K and V8, they always wandered 
a little bit when going up or down grade.
5. Wouldn't it be better to use a GPS unit?  As I understand, they aren't 
great at measuring acceleration due to the sampling time, but for an 
instant measurement at a constant speed, they should be good.

Another good site I like for looking for new wheel/tire combos is: 
http://toy4two.home.mindspring.com/tire.html. I like it because it lists a 
variety of combinations within set parameters (tire width, wheel size, and 
speedo error).  As it uses 70 mph as a base, I use .7 mph if I want to stay 
within 1% error, 1.4 mph for 2%, etc.

Later,
Arryn


At 02:33 PM 11/27/2003 -0500, Ingo Rautenberg wrote:
>You may also bear in mind that Audi's have traditionally been a bit
>"optimistic" when it came to speedometer readouts (at least in the past).
>Wouldn't surprise me in the least if your indicated speed with stock
>rubber/wheels was 5% optimistic...
>
>Ingo
>
>----- Original Message -----
>From: "JOSEPH TICHY" <JOSEPHTICHY7448 at msn.com>
>To: "Bernie Benz" <b.benz at charter.net>
>Cc: "200q20V mailing list" <200q20v at audifans.com>
>Sent: Thursday, November 27, 2003 2:37 PM
>Subject: Re: Wheels/Tire Combo ?
>
>
>Bernie:
>
>I arrived at my speedo result comparison based on this calculator, available
>at Ned's Intended Acceleration website:
>
>http://www.miata.net/garage/tirecalc.html
>
>A very crude method, but nonetheless effective for an "idea" of what the
>difference would be.
>
>Your suggestion is a good one, and in my state there are 5 mile check points
>on the interstate in various locations for just this purpose.  I will
>certainly have to cross check any differences when the wheels are finally
>bolted up.
>
>JOBE
>   ----- Original Message -----
>   From: Bernie Benz
>   To: JOSEPH TICHY
>   Cc: 200q20V mailing list
>   Sent: Thursday, November 27, 2003 11:13 AM
>   Subject: Re: Wheels/Tire Combo ?
>
>
>
>   > From: "JOSEPH TICHY" <JOSEPHTICHY7448 at msn.com>
>   >
>   > I entered a comparison of the stock tires and the 16's with 225-50-16
>tires
>   > and it appears to be a wash for the speedo:  60 stock will read 59.3.
>[the
>   > speedo reading will be 59.3 too for the A4 tire size of 215-55-16, which
>makes
>   > sense seeing how the 225-50 series is merely a plus one over 215-55]
>   And just how do you easily check the speedo readings with different tire
>   sizes?
>
>   A more considered method might be to check the odometer over a 5 mile
>   roadside measured course.  You may well find that the stock tire size does
>   not give the correct odometer reading, hence you can adjust your new tire
>   size to correct this error.  Once you get this right, you can then measure
>   the speedo time error over the same course at an indicated 60 MPH and
>   calculate the % error.
>
>   Bernie
>
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