<VV> 175 vs 185

[Roger Gault]

I'm guessing Bob's numbers are for a 75% or a 80% profile tire.
Of course none of the endless rev/mile nitpicking makes all that much
difference.
At 70 mph, the difference in speedometer reading between a 843 revs/mi tire
and a 863 revs/mi tire is about 1.7 mph.  It's unlikely that a 40 year old
speedometer is accurate enough for that to matter.

Not to mention that there are two different speedometer calibrations 833 and
853 revs/mile, depending on what tires were original on the car.

Bryan Blackwell's site seems to be down at the moment, assuming this is only
a temporary disaster, there's a good discussion on there.
http://autoxer.skiblack.com/

Also, there's quite a bit of difference between different manufacturer's
revs/mile for the same size tire.

The tire situation is getting grim.  If you want much footprint, and you
don't want to run huge rims with rubberbands wrapped around them, you're
pretty much screwed these days.  I had to give up on 13's and go to 14's - I
should have skipped to 15's.  I used to run 195/70-13s on the front and
205/70-13s on the rear (whitewalls).  Forget that these days.  I'm currently
running 195/65-14's.  I stuck some 215/70-14's on the back - ran quiet on
the highway, but was a real dog off the line.

As for general performance, I'd suggest as much rubber on the ground as you
can get.

Roger

----- Original Message ----- 
From: <BobHelt at aol.com>
To: <bowtieguy at cox.net>; <virtualvairs at corvair.org>
Sent: Friday, September 01, 2006 11:56 AM
Subject: Re: <VV> 175 vs 185


>
> In a message dated 9/1/2006 9:29:33 AM US Mountain Standard Time,
> bowtieguy at cox.net writes:
>
> I  thought the 175 was closest in size diameter to the 650:13 from past
> discussions for speedometer accuracy. What did I miss?
>
> Bob  Vukas
>
>
>
> Hi Bob,
> As with all things in life, one must define his terms. Otherwise we start
> comparing apples and oranges.
>
> First comparisons between stock (came on the car) tires and replacements
can
> generally be based on one of two characteristics (ignoring things like
ride
> and cornering).
> These are 1) comparison of tire measurements of tires mounted on rims and
> filled with air.These wsould include diameter and circumference, etc.
>
> Second characteristic 2) would be how many revs per mile the tires go when
> mounted on the car at say, 50 mph. This would affect speedo accuracy and
other
> things.
>
> These characteristics are entirely different because of the change from
bias
> tires on the 1960s and radials of today.
>
> So here are the answers:
>
>                                                 6.50-4ply        6.50-2ply
>    175          185
> diameter  (Unmounted)            24.3                24.3
24.1
>      24.6
> revs per mile  (RPM)                 853                 864
> 863         843
>
> So you can see that 175 tires are similar the the 2-ply in RPM but the
185s
> are closer to the 4-ply 6.50s.  So the choice now becomes less distinct.
> Similarly, the 175s are smaller in diameter than the 6.50s and the 185s
are
> larger. So which do you choose? This affects how the tires fill the tire
well  and
> how they "LOOK" when on the car. Most people choose the 185s for
appearance
> and for being closest to the original 4-ply tire RPMs to keep the speedos
> reading close.
>
> Regards,
> Bob Helt.
>
>
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