Tires sizes and effect on speed/load ratings?

Question

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.

y_p_w · Accepted Answer

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

Brent P · Answer

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.

phaeton · Answer

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

y_p_w · Answer

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

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.

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.

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.

ray · Answer

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

Tires sizes and effect on speed/load ratings?

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