Tire Speed Ratings

Searched this but didn't really find much.

In the standard Prius tire size, there isn't much variation in the speed ratings available. From reviewing the TireRack offerings (plenty) it appears that the vast majority of tires available are S, T, or H rated. Obviously, from a safety point of view, these are all perfectly well serviceable. I'm wondering whether the speed rating has any effect upon (or correlation with) the rolling resistance, and thus, ultimately fuel economy. For example, all other things being equal (I know, they never are...), would a V rated tire, be better or worse than an H rated tire? I've looked at one chart of rolling resistance data, and if there's a pattern in it, I haven't quite discerned it yet. On one hand, I'd think that the generally stiffer sidewalls of the higher speed tires would be good for us. OTOH, the higher speed tires also usually have "stickier" compounds, which I'd think would hurt FE. Before I reinvent the wheel (oooo, bad pun....), anyone dug into this question?

 

Thanks guys, I appreciate the quick and thoughtful responses. I realize that the speed rating is not a "stand alone" fact, but rather is itself the result of a number of design parameters of the tire, which themselves may or may not bear upon rolling resistance. Until I got a Prius, two weeks ago, I hadn't given much thought to controllable items that you can use to shave your mpg. In fact, since I owned a G35 and a Sequoia, I tried not to think much about mileage at all... Yep, the "tree" is getting "chopped" soon too...

Another thought/question on this: As I've been ruminating on tires (my car has only 15k miles, so it's not a rush thing), it seems that two (the two?) major factors in RR would be sidewall stiffness and flex, as well as the "stickiness" of the tread compound. From what I've seen (or not seen), it appears that neither factor consistently "dominates" a given tire's ultimate RR. Are there any other factors I'm missing that bear significantly on RR (whether or not the're easily quantifiable by the consumer)?

A parting observation: have any of you noticed how incredibly thin the sidewalls are on the Goodyear Integrity? We had on 01 Highlander for a while, and it too came with Integs, which I dumped after about six months for some LTXs. The sidewalls were really floppy and seemed about as thick as a rubber raincoat, if that. Surprised, I checked the stack of MI-LTXs they were installing and the walls were much more substantial. I wonder if there's a RR clue in those really, really thin sidewalls?

 

Sidewall "flex" is an issue because each time a given point on the tire rotates down for its next "turn" at being in the contact patch, the car pushes the sidewalls adjacent to that point outward, stretching and "bending" the tire structure as it does (it of course snaps back as that point rotates out of the patch). This, of course, builds heat in the tire, and is why underinflation is so critical, especially in already-tippy SUVs. Too much heat generated (and flexing beyond what the tire structure is meant to endure), and ka-boom. Ask yourself where that tire heating energy is ultimately coming from (your gas tank, of course). Conversely, as Prius owners overinflate (the oft seen 42/40 advice), they are keeping the tires sidewalls from flexing as much as they would at lower pressures, thus reducing resistance (the 42/40 thing also probably reduces the patch size to a very small degree, further lowering friction). With each rotation of the tire, the car must "push" the tire through the flexing of its entire circumference.

 

tnthub:

Absolutely true that radials are more efficient than bias ply tires, which is why bias ply tires mostly disappeared from passenger car use a couple decades ago. But what counts is not the absolute amount of flexing a tire does, but rather, how much energy it takes to flex the tire. Radials take a lot less than bias ply tires do, and of course, they are of very different internal construction. But the fact remains that as the car rolls forward (or backward too, I suppose...), it is continually "pushing through" the flex resistance of the tire, again, be it a radial or a bias ply type. And for a given tire, by increasing its pressure, you're stiffening the sidewalls, reducing their flexing, and decreasing their rolling resistance. This is why it's so important to keep tires at or above "normal" to achieve good fuel economy. Underinflation increases sidewall flexing, leading to much greater amounts of engergy being wasted, much of which gets converted to heat (so much so that the heat can cause catastrophic failure). Think about it from the other side: as you watch a tire rolling down the road (attached to a car, we hope), where does the energy to constantly flex and unflex the side of the tire come from. Now, comparing the same radial tire, not radials to bias ply tires, which will take more energy to push along, the one that's flexing a lot, or the one that's flexing less?

 

<div class='quotetop'>QUOTE(tnthub @ Aug 13 2006, 04:19 PM) [snapback]302662[/snapback]</div>

Gee, I wonder why those guys aren't using Goodyear Integritys??? Seriously, cool video -- shows nicely what happens under super extreme torque loading.

Wilco:

Ooops, I hadn't considered of the weight factor, and how the added structure in a higher speed tire can add more weight. The GY Integs are pretty light at 17 lbs for our size, though you can have IIRC, one of the Kumho models at only 15 lbs. There are a couple same-sizers that are 20# or more. More food for thought. Thanks.

 

<div class='quotetop'>QUOTE(Blackfang @ Aug 14 2006, 08:48 PM) [snapback]303287[/snapback]</div>

Shoot there 'fang, if you're a member of the "stop tryin' at 30 mpg" club, you're definitely in the wrong forum! I didn't ditch a perfectly good (well, almost perfectly good) G35, and replace it with a Prius, just to get "good enough" mileage. And let's face it -- the engineers who designed this car put that big MFD, right smack in the middle of the dash for the same reason that matadors wave red flags in front of bulls -- to provoke a totally predictable, totally unavoidable response. The bull is goaded into a singlemind focus on one thing -- driving a horn as far through that pesky matador as he can. We Prius drivers are likewise goaded to focus on one thing -- getting every last millimeter of travel out of every last drop of gas. I'm already so into it that -- gak -- I might even buy another set of Integritys. :blink: Semper Fi.

 

No Bob, respectfully, you missed my point, on two levels. First, the higher speed rated tires (V for example, at up to 149 mph) are noted for their stiffer sidewalls, designed in part, to combat the accumulation of heat, which in turn, can lead to catastrophic failure. Since stiffening the sidewalls is exactly what a Prius owner accomplishes by slightly overinflating (the 42/40 thing), I raised this question to see if there was any consensus developed, or pattern apparent, of higher speed rated tires helping mileage. I am, as stated before, well aware that other features of higher speed rated tires can and do detract from mileage.

Second, the post was really meant to be an exercise in humor. I mean, really, you don't think that I try as hard to increase my mileage as the bull does to kill the matador, do you? (edit - the 30 mpg figure btw, came from his post; I get ~48 mpg avg) The point was to poke a little fun at those of us, obviously including myself, who do spend too much time thinking about this. So come on, relax a little and enjoy. I'm sorry if you didn't find it amusing ... I'll try harder next time.

 

C'mon 'fang, relax, back on the meds, please! If you look at the beginning of the thread, I was simply asking the question, I was not claiming that higher-than-standard speed rated tires would increase mpg. The question arose in my mind because one of the characteristics of higher speed tires is in fact stiffer sidewalls. But of course, they have other characteristics that can, and I guess do, exact a mileage penalty (e.g. stickier tread compound). I'm new to the Prius community, and certainly don't see many other Prii except mine (except in passing), so what's the problem with asking? I've been averaging 48.5 in city driving for the last two weeks, 11.5 less than EPA, and I'm not disappointed. But one of the prime reasons for getting a Prius is to maximize mileage, so really, you shouldn't be surprised that owners try to, and discuss, any way they can find to shave off another mpg or two. Are we any more silly than the 'vette or 350Z guys who obsess about shaving tenths of a second off their quarter mile times?