I need to replace my tires on 2004 350Z...specs want 235/50 R17 96W on REAR and 225/50 R17 94W on FRONT. Costco have smaller front tires per specs, but only have 235/50 R17 96H for the rear tires...the difference being H vs W. Can the REAR tires have a lower speed rating than the FRONT tires...or can I put the smaller (225/50 R17 94W) on all four tires?

 

^^ You are fine installing tires with different speed ratings front to rear.

Correct sizing is more important than speed rating.

 

Interesting... two posts on the idiocy that is Costco Tire sales in less than a week.

FWIW, it is ILLEGAL for a tire shop to install lower speed rated tires on a car than what was originally specified by the manufacturer.

Period.

So often I used to hear "Can't you just sell me that tire...? I mean, it's $20 less than what you want to sell me. I don't drive 130mph!"

"Nope, sorry."

 

Sorry Spike, not true.

Well, technically, mixing is fine ONLY if, say, the fronts are W or Ys and the rears are "only" Zs.... the Z meaning the lowest common denominator (or minimum specifications.)

 

I’ve never given any significance to speed ratings. That’s because it’s an imperfect science without any reliable parameters or real-world repeatable testing.

And… the actual speed related to the speed ratings is ridiculously high. The speed rating is a quantifiable value, but without reliable testing statistics or values.

 

I won't get into the science of tire testing here - to keep with the originally posted question - but suffice that, once again, it is illegal to sell/install a tire that doesn't meet the OE requirement. There's many reasons for this but yes, liability and safety, etc. all come into play here.

Personally, it makes sense to me as speed ratings not only designate a certain confidence level in my tires but also, a higher speed rated tire is engineered and designed with a generally higher level of performance. Ply type/materials and the way in which they are layered through the tread area and more importantly, how they tie into and integrate with the sidewalls also yields the type of performance (immediate response and ultimate grip) I want when I impart cornering forces on it through the steering wheel.

I may never take my car into the stratospheric speeds at which a Z, W, Y tire can theoretically be taken but I know that I have reserve left in my tires at all speeds; moreso than, say, a 130mph H-tire and S-rated 113mph tires will never see the wheelwells of my Zs.

 

Those are good points.

I’ll add this: If you plan driving your car in excess of 100 mph, you need to pay attention to the speed ratings on tires you install.

If you use your car a daily-driver and do not drive over 85 mph, the speed rating of your tire has much less importance than your tire size and the type (summer performance, all season, etc.) of the tire you mount on your car.

 

*****WARNING: VERY LONG and TIRING *****
(Sorry for bad pun but you're warned to consider moving on to another post if tires aren't up your interest alley.)

Spike et al,

I don't mean to beleaguer this discussion any more than it needs to but oversimplifying tire choice is just not a good idea.

Earlier in the conversation, the point was made about the legality of installing tires that were below the vehicle manufacturer's recommendation. OK, I don't recall if this is something that is stipulated by an actual LAW. However, within the tire industry, it is something that major tire companies/retailers take very seriously and therefore using the word "law" might be a little strong... let's just call it SOP by most major retailers.

And there's a lot of good reasons that I started to elucidate on above but this further discussion begs even more points be made. I'll only bulletize so as to not draw this out (even more) unnecessarily and further away from the OP's 1st question.

Facts to consider tangentially related to the originally posted question:

- It was said that there are no measurable standards by which tires can be tested empirically. This is just not true. When I managed a shop in which tires were about 65% of our business, I wanted to see just how much things had changed since I was originally in the biz 40 years ago so I did some thorough research and boy, did I learn how much things changed!

Tire testing is stipulated and governed under rules set by NHTSA. They, in conjunction, with an international body (can anyone say "U.N."), worked to establish a set of rules for all automobiles, trucks and anything that operates on public roads. That set of rules is called the FMVSS or Federal Motor Vehicle Safety Standards.

All aspects of vehicle safety from passive to active safety systems to emissions to chassis, fuels and so on are covered under this set of rules.

Tires, being the critical link between a car and road, falls under that pervue as they should.

I won't get into the actual testing - which is massively oversimplified on any tire website you read about "speed ratings". The actual testing of tires, while subjected to the tests you can read about anywhere, is far more detailed and elaborate. If one wishes to obtain more info on this than I can or will explain, look up the specifics on the NHTSA website.

I guarantee you that you will be amazed at the extensiveness and thoroughness of the testing done as compared with the simplified "spin it against a drum for 10 minutes and see if it falls apart" explanations easily found.

The actual testing described by NHTSA goes all the way into the process and holds out for your view, the exact criteria upon which a tire is deemed "safe" after all of the hot/cold testing is done; including visual and x-ray examination, dissection and more.

- Speed ratings are also - almost - directly related to a tire's ability to accept and reject HEAT. Sure, there's a heat rating in the UTQG rating on the sidewall. An "A" heat rating being the top rating. Most performance tires are easily "A" temperature rated as assigned by the manufacturer.

However, the real question is "How much over and for how long can a tire last at/above those 115degF temps that an "A" rating indicates?"

The manufacturers know this and so they MUST engineer tires that are designed to run 150mph+ to be able to withstand the type of heat generated by sustained high speeds. So, while there's no DIRECT statement of sustainability, rather, implied, the manufacturers design their highest speed rated tires to run at a higher maximum temperature than "pedestrian tires" found on Kias or Tercels.

- Final point in this unintentionally lengthy bit, tread design and compounds used for the tire's tread and shoulders are also designed and maximized through tire engineering so that they are able to run at those high speeds and be able to withstand the heat buildup above but also to maintain some semblance of stability at speed.

So, while there's no such thing as "tread standardization", you can bet that a tire designed for sustained Z+ speed running will also have a lot more in reserve when it comes to it's ability to do all the things its supposed to do well at all speeds but particularly at high speeds.

Things such as:

- "Not come apart" (having a tire fall apart/separate isn't fun at ANY speed)

- Reject heat (maintain a relatively livable temperature) to avoid catastophic failure from belt separation, sidewall collapse, etc.

- Not squirm around (to avoid loss of control)

- Channel water at high speeds through proper tread/sipe design cuz hydroplaning is a bish!

And more...

==========

I could go on but.... zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz.

I will say this, getting back to the first thread starter question, there's an unwritten "shop saying" (based in fact and is mandated by many retailers, apparently not Costco) that IF one were to mix speed ratings on a vehicle, the LOWER speed rated tire needs to go on the FRONT of the vehicle (regardless of which wheels are driven) so as to preclude oversteer conditions.

THAT right there says something..... an H rated tire up front will "wash out" (or cause understeer) more than a Z rated tire up front. That indicates a lower level of adhesion/traction at the front in this scenario.

Then by inference, that leads to the anecdote/hypotheses that the "H-rated tire has less performance available" then a Z-rated tire... underscoring what I mentioned a few posts above about higher speed ratings GENERALLY leading to higher performance.

(I realize that's just not true in the real world because there are many tires carrying Z+ ratings but suck nonetheless; but I'll stick to the correlation before I try and save a couple bucks by going with a lower speed rated tire.)

Anyone make it this far? Laff.....

 

^^ I did "make it that far." And, I believe your posting has correct information.

My advice here is educating owners on tire and wheel requirements established by the companies and the manufacturers. There is so much misinformation in this realm.
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EDIT: The tire and wheel size (and wheel offset) are not relevant to this discussion.