Tires sizes and effect on speed/load ratings?

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y_p_w - 14 Nov 2006 16:43 GMT

I recently bought a set of Bridgestone Potenza RE960AS
Pole Position tires in stock 205/55R15 (W speed rated)
for my 2004 Subaru Impreza WRX. As I was scanning
through the Tire Rack (that's not where I got my tires)
listings for this tire, I saw the different speed
ratings for different sizes of the same model, or even
sometimes the same size in the same model. My size is
just where RE960AS goes from H speed rated to W.

So is there anything different about the construction
or materials that would make mine W rated? Maybe just
certain dimensions or maybe there isn't that much of a
difference? I'm really curious about tires where the
same model/size is sold with two speed ratings. The
Michelin Pilot Exalto A/S was about $40 more for a V
rated tire in my size compared to their H rated version.

I like reading the Tire Rack reviews, but sort of take
their numerical rating system with a grain of salt.
People are assessing these tires on different cars with
different sizes and different expectations. I wonder
when some Summer tire that I assume would have some
incredible grip gets a higher dry traction survey
rating than a better than average all-season
performance tire.

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y_p_w - 14 Nov 2006 16:45 GMT

> I like reading the Tire Rack reviews, but sort of take
> their numerical rating system with a grain of salt.
[quoted text clipped - 4 lines]
> rating than a better than average all-season
> performance tire.

Sorry - I meant "why would a Summer tire get a lower
rating than an all-season tire?".

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Brent P - 14 Nov 2006 17:00 GMT

> Sorry - I meant "why would a Summer tire get a lower
> rating than an all-season tire?".

Because some summer tires might not be as good or you said it yourself,
different expectations. Someone's idea of great grip for an all season
might be just average in that same person's mind for a summer tire.

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phaeton - 14 Nov 2006 22:37 GMT

> > I like reading the Tire Rack reviews, but sort of take
> > their numerical rating system with a grain of salt.
[quoted text clipped - 7 lines]
> Sorry - I meant "why would a Summer tire get a lower
> rating than an all-season tire?".

A lower rating in which regard? At first you were talking about speed
ratings, but then it seems you switched to traction ratings.

Speed ratings have nothing to do with tread or tire size. It's all
about tire carcass construction- using different amounts and types of
materials to allow the tire to withstand higher amounts of centrifugal
forces and temperatures at speed.

Traction ratings tend to be more related to the chemical composition of
the exterior, tread design (how much tread area is on the road wet or
dry) etc. Generally more 'grippy' performance tires are softer, and
high-mileage/economy/all season tires are of harder compound, but there
are some exceptions.

And of course, (like you said), the vehicle they are on affects
everything too.

-phaeton

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y_p_w - 14 Nov 2006 23:37 GMT

> > > I like reading the Tire Rack reviews, but sort of take
> > > their numerical rating system with a grain of salt.
[quoted text clipped - 10 lines]
> A lower rating in which regard? At first you were talking about speed
> ratings, but then it seems you switched to traction ratings.

Just a "while I'm here" additional comment.

> Speed ratings have nothing to do with tread or tire size. It's all
> about tire carcass construction- using different amounts and types of
> materials to allow the tire to withstand higher amounts of centrifugal
> forces and temperatures at speed.

I'm just wondering if there's actually any difference when the same
model of tire comes in different sizes that should result in a
different speed rating. I would think that part of the ability to
resist temps and/or dissapate heat has to do with the tread
compound. Again - there was that Michelin Pilot Exalto in
205/55R15. I'm curious as to what justifies $143 for the V
rated version vesus $103 (reg price) for the H rated version.

> Traction ratings tend to be more related to the chemical composition of
> the exterior, tread design (how much tread area is on the road wet or
> dry) etc. Generally more 'grippy' performance tires are softer, and
> high-mileage/economy/all season tires are of harder compound, but there
> are some exceptions.

I always wondered about that. Many of the super-grippy high
performance Summer tires I've seen are harder than all-season
treads. I've even heard that many of these Summer tires can be
ridiculously hard at cold Winter temps. It seems to run counter
to the logic of Summer tires being softer for better grip.

> And of course, (like you said), the vehicle they are on affects
> everything too.

Yep. I like the Tire Rack reviews, but I think a better course
would be to actually read the reviews from people with similar
vehicles and/or tire size.

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ray - 15 Nov 2006 02:47 GMT

> I always wondered about that. Many of the super-grippy high
> performance Summer tires I've seen are harder than all-season
> treads. I've even heard that many of these Summer tires can be
> ridiculously hard at cold Winter temps. It seems to run counter
> to the logic of Summer tires being softer for better grip.

ahh... but summer tires with a "high" like Y speed rating need to be
able to withstand the heat generated from sustained high speed running,
and thus have a hard tread compound when cold.

my Kumho ECSTA MX's on my Trans Am are like this - they need some heat
in them to have any grip - if I drive the car when it's say 5 above
freezing it's like driving on ice until they have driven about 10 miles.

Ray

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