Re: [alfa] Formula for calculating correct tire size

[Richard Welty <rwelty@xxxxxxxxxxxxxxx>]

On Thu, 08 Jul 2004 14:47:35 -0700 alfacybersite <acs@domain.elided> wrote:

> Read Jeff Greenfield's post first. Yeah, I thought S and SR sure looked 
> like speed ratings. I also figured the original sidewall height of the 
> '69 Spider's sidewall was probably 70. 

OE? certainly not. in 69, the OE aspect ratio would have been 80,
and so the sidewall height would have been tread width * 0.80

>If that is the case, how come the 
> calculator gives 23.09" diameter for a 165/70-14 (presumably new) tire 
> and the well used 165/SR-14 tire I have measures 24"? 

the discrepancy doesn't exist, because you guessed wrong about the
OE aspect ratio.

>Or are these 
> measurements purely arbitrary and one manufacturer says, "Hey, let's 
> make our 165/?-14 tire 1" taller than our main competitors?" 

there is always some variation in rolling radius due to manufacturing
variations, and probably due to bean counter derived cheapo tires
which intentionally have less material to cut costs. you're not going to
see a whole inch in diameter, though.

> What owner 
> ever knew if he had a 70, 78,  80 (or?) series tire in the "good old days"?

in the good old days, everything _was_ 80 or 78. a 165-14 Conti
was more-or-less interchangable with a 165-14 Semperit, more-or-less
interchangable with a 165-14 Michelin, and so forth and so on.

richard
-- 
Richard Welty                                         rwelty@domain.elided
Averill Park Networking                                         518-573-7592
    Java, PHP, PostgreSQL, Unix, Linux, IP Network Engineering, Security
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided