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RE: Tires

As I recall,  the typical tire has a tread depth of about 12/32 inch. At
2/32 they are considered "bald". Taking away 10/32 of the rolling
diameter from a 15 inch 215/60 reduces the rolling distance from ~2007mm
to ~1982mm or about 1.25%. This would be about .75 mph difference at
60mph. So the difference in a new tire and an old one is only about
3/4ths the difference of going from a 205/55 to a 215/50. 

>From: 	Peter Henriksen
>Sent: 	Wednesday, July 10, 1996 9:44 AM
>To: 	'quattro@coimbra.ans.net'; 'jackr@internet.kronos.com'
>Subject: 	RE: Tires
>From my calculations (read: YMMV :-):
>Tire size	Total diameter	Rollout		Change from stock
>205/55	631.9mm	1985.1mm	N/A
>215/50	621.4mm	1952.1mm	1.7%
>225/45	608.9mm	1912.9mm	3.6%
>Formula being: ((2*((<profile>/100)*<width>))+(<wheel size>*25.4)) to
>get a result in milimeters for the diameter and then 2*pi*r to get the
>rollout (pi somewhat equal to 3.1415).
>Did I just do something stupid or is that right? It's still pretty
>early, I haven't had nearly enough coffee and I haven't done anything
>like this since college...
>On that note, how much of a difference is generally acceptable in
>rollout before the speedometer and odometer start getting way off? Where
>do you go to get the instruments recalibrated?
>- peter henriksen, peterhe@microsoft.com, issaquah, wa
>  91 200qw
>  94 acura legend gs
>  80 mazda 626
>>From: 	jackr@internet.kronos.com[SMTP:jackr@internet.kronos.com]
>>     >>One question. Has anyone put new tires on yet? I am planning on 
>>     >>putting 225/50 R16 Dunlop D40M2's this week. They have the same 
>>     >>circumference as the 205/55's and supposedly enough clearance. Has 
>>     >>anyone heard differently?
>>     My understanding of tires is that a 215/50 or 225/45 would have the 
>>     same circumference as a 205/55.