Quattro Spare Tire Myth Uncovered
03-03-2003, 05:51 PM
#1
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Quattro Spare Tire Myth Uncovered
Myth: Using a NEW spare tire with OLD worn tires will damage the Quattro.
Truth: If you are using the same tire size it does not matter - it will not hit the 3% limit.
The math:
Old 235/45 R17 tire with the legal min tread 2/32"
New 235/45 R17 spare tire with new tread 10/32"
The diameter of a NEW tire is:
+105.75 mm (235mm * 45% = 105.75 tire sidewall)
+105.75 mm sidewall applies twice (top/bottom)
+431.00 mm (17" rim)
=======
=642.50 total diameter = 2018.41 mm circumference
So the diameter of a used OLD tire is:
+642.50 new tire (see above)
-006.35 mm (lost tread 10/32 - 2/32 = 8/23 or 1/4")
-006.35 mm (sidewall loss applies twice)
=======
+629.80 total diameter = 1978.51 mm circumference
629.80 Old
/642.50 New = 98%
So there is a 2% difference in the diameter/circumference.
And from what I have read it takes over a 3% difference to interfere with Quattro AWD.
Truth: If you are using the same tire size it does not matter - it will not hit the 3% limit.
The math:
Old 235/45 R17 tire with the legal min tread 2/32"
New 235/45 R17 spare tire with new tread 10/32"
The diameter of a NEW tire is:
+105.75 mm (235mm * 45% = 105.75 tire sidewall)
+105.75 mm sidewall applies twice (top/bottom)
+431.00 mm (17" rim)
=======
=642.50 total diameter = 2018.41 mm circumference
So the diameter of a used OLD tire is:
+642.50 new tire (see above)
-006.35 mm (lost tread 10/32 - 2/32 = 8/23 or 1/4")
-006.35 mm (sidewall loss applies twice)
=======
+629.80 total diameter = 1978.51 mm circumference
629.80 Old
/642.50 New = 98%
So there is a 2% difference in the diameter/circumference.
And from what I have read it takes over a 3% difference to interfere with Quattro AWD.
03-03-2003, 06:11 PM
#3
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Not worth it IMHO.
And even if they are not directional you don't want to reverse the direction of the tires. So you could only rotate it on one side. I would just leave the spare in the trunk for when you really need it or when you get a flat. (It will happen - just give it time.)
If you want to save money:
When you get new tires swap out the spare with the best existing tire. And buy 3 new tires same brand/model.
If you want to save money:
When you get new tires swap out the spare with the best existing tire. And buy 3 new tires same brand/model.
03-03-2003, 06:52 PM
#7
I am not totally comfortable with April's figure. Swap a new tire? Ok, but count how many extra revs
any given wheel will rotate many more times per mile at a 3% difference. It seems like way too much for a differential to suck up.
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03-03-2003, 06:59 PM
#8
AudiWorld Expert
that's basically what I had in mind too...
I'd use a brand new spare tire as a temporary solution, but I'm still hesitant just to leave it on forever. Who knows, maybe I'm just too paranoid. After all, I don't really know what the torsen can and cannot withstand.
