G1 tire requirements

I've been reading a lot of stuff concerning G1 tire selection, like john1701a's user guide, which contains a lot of tire information but seems a little dated at the moment (in terms of what tire models are currently available).

The issue of load rating and inflation hasn't been completely clear to me. John's user guide has that extra-load (XL) tires are required to support the car's weight at the OE spec inflation of 35/33 psi, but that standard-load tires can be used by inflating above 35 psi to increase their load capacity. But it doesn't cover how to be sure of the adequacy of any given combination of tire and inflation pressure. It says that 1,102 lbs load capacity is needed for the front end, but I noticed a lot of tire manufacturer web sites will happily offer me tires with 1,019 lb rated loads, and I didn't find any information on how to compensate with higher pressure.

So I started trying to learn all the basic stuff I didn't know. The most helpful reference I found was a Bridgestone/Firestone Tire Replacement Manual (which I'll call TRM).

First, I couldn't find anything in the Toyota manual that mentioned "XL", but the placard in my glove box does say: P175/65R14 84S. 84S is what's called the service description, and has two parts: S is a speed symbol (never exceed 112 mph) and 84 is a load index (1,102 lbs when inflated to 41 psi or higher, which puts it in an "XL" category). (TRM pp. 5-8, B4)

Also, the P in P175/65R14 makes this a "P-metric" tire type. It turns out that 175/65R14, without the P, is also a real tire type ... a different tire type, with different load characteristics. (TRM p. 9.) Isn't this fun?

But wait. The table on page B4 gives a capacity of 1,102 lbs but only at 41 psi or higher. At the OE spec 35 psi, it's only 1,019 lbs for the front (and at 33 psi, 994 lbs for the rear). Aha! That's why manufacturers are willing to show not only 84S tires but also 81S models with 1,019 lb capacities.

Are 1,019 lbs front / 994 lbs rear reasonable capacities for a G1 Prius when loaded to the max? Well, the Gross Axle Weight Ratings on my door pillar are 1970 lbs front (985 per tire) and 1685 rear (843 per), so yes. In fact that gives a generous margin, 17.9%, for the rear, and a smaller margin of 3.5% for the front. (That also makes sense because if you do overload a Prius, the bulk is likely to be in the rear.)

But what inflation pressures would I need in an 81S tire to equal the 1,019 front/994 rear capacity of the 84S OE tire at OE pressures?

Answer: OE pressures! In the table, the 81 and 84 capacities are equal at all pressures not exceeding 35 psi. The only difference is that the 81 tire maxes out at 1,019 lbs capacity and increases no further from 35 psi up to its maximum inflation pressure, where the 84's capacity continues to increase to 1,102 lbs at 41 psi, and then increases no further up to its max allowable pressure.

That's not just a quirk of the table, it's how the rating system works; see TRM p. 11, "Reinforced or extra load tires require higher inflation pressure to attain the added load capacity." At 35psi or below, they don't have any more load capacity than the standard load tire at the same pressure. And all types of tires can be marked for a maximum inflation pressure, which can be well above the pressure that gives their max load capacity, and any additional pressure doesn't increase the capacity further.

This kind of goes against earlier wisdom, which said that if you replaced an 84 with an 81 you should increase the pressure beyond 35 to match the capacity - but at 35 psi the capacity already matches, and increasing above that only makes a difference for the 84, not for the 81! (And the difference doesn't matter because the capacity is already adequate.)

But increasing the pressure still can have MPG, ride, and tread life effects.

That does leave open the question of why Toyota specified 84S when they did not specify a higher pressure to have the benefit of the extra capacity. I can't answer it yet. Somewhere I read that tall or narrow vehicles may specify XL tires just because the reinforced sidewalls help control sideways motion. Maybe they liked that aspect.

There is still an uncomfortable hole in my knowledge. Even though TRM includes this whole table in appendix B with load capacities for P-metric tires, it also says (p. 9) that for P-metric tires a load index table isn't the whole story! It mentions (p. 20) using data books like the "Bridgestone P/LT Data Book" which maybe have tables like this for specific tire models. I don't know because this book doesn't seem to be online and I haven't found one. If that's really true, we should be revisiting this by looking up the OE Potenza RE92 84S in the data book to find its exact capacities at 35/33, and when using an alternate replacement tire, look in its own manufacturer data book to find the necessary inflation.

I don't know yet how much of the data is standardized and how much is left up to any manufacturer's data book. For example, when I look up G1 Prius at Dunlop I get several 81S tires that show load limits of "1,019 lbs at 44 psi". But I don't know if that means you really need 44 psi to get 1,019 lbs capacity, or 44 is the max inflation and you get 1,019 at 35 or above. Their web site just doesn't state it clearly enough, so maybe I would need to look in a Dunlop data book to be sure. Unless I can find an authoritative document from TRA (which sets the P-metric standards) that makes it clear what the standard is.

Can't anything be simple?

-Chap

 

Well, as a guess, that has a sort of plausible sound - but notice you can make the opposite guess sound plausible too: given that the two tires have equal load capacities at the spec'd inflation, the one with the 84 index is going to have more rigid sidewalls and provide less driving comfort.

Since the two guesses have opposite conclusions, they can't both be right, and I don't know enough about the subject to decide which is right (if either; neither one has to be). Which is kind of the whole point: I'm trying to find the references where I can actually learn enough to confidently compare tire alternatives, without guessing.

-Chap

 

Yup, exactly the thing I don't like about it. And there are so many choices available in 81 load index, and most of the tire vendors' databases happily offer them when you query for a G1 Prius. When I first saw that I thought "ugh, how irresponsible!" but if what I've found so far is right and they have the same capacity even at the OEM inflation (the higher capacity of an 84 is only realized at higher pressures), then it makes sense that the automated databases are including them.

I still haven't hit the library to see if what I found in the Bridgestone/Firestone book is actually standardized, or specific to Bridgestone/Firestone. I would have to be sure it generalized across tire makes, before relying on it.

-Chap

 

Heads up!

Heads up ... pretty much everything I've said earlier in this thread applies only to P-metric tires (the ones whose size starts with a P), and is dead wrong for Euro-metric tires (the ones with no P).

Oy.

-Chap

 

Hey Mike,

Can you remember where you saw that list? I spent waay too much of yesterday looking around for just such a list - the closest recent thing I found was a list of tires CR thought was good but without actual numbers.

Oh, hmm, I had also seen this, about 5 years old but does have a number for a B381 in 185/70R14 (ugh, these guys are careless with the 'P' too). .0062.

-Chap

 

The dope on load/inflation minimums for P-metrics

Finally got to the library....

It turns out the standard specs for P-metric tires come from this outfit called the Tire and Rim Association, and they're published in the "TRA Yearbook". The library has 1996 through 2004, so I got to look through several years spanning before to after the era of the G1. The details I was interested in were pretty much unchanged over those years.

One thing I read there and hadn't ever thought about before is that rims are also marked for maximum pressure. On mine that must be the "45" that comes after "TOYOTA 14 x 5 1/2 JJ". So even if a tire has a maximum pressure of 50 psi, the limit is 45 because of the rim. Good to know. Edit: I was mistaken on this point, and corrected by tochatihu here, as I acknowledged here. Pressure markings are only required on non-passenger-car, non-one-piece rims, and the "45" on a Prius rim means something else.

The load capacities for P175/65R14s are standard, either "81" (981 lbs at 32psi, 1019 lbs at 35psi), or "84" ("XL", 981 @ 32, 1019 @ 35, 1058 @ 38, 1102 @ 41). (From Table P-1, in the 2004 yearbook on pp. 1-12 and 1-13). You can inflate either kind of tire further, up to its labeled max pressure, but with no additional capacity, so an 81 is still limited to 1019 lbs even if inflated past 35, and an 84 is still limited to 1102 even inflated above 41. At 35 and below, there's no capacity difference between an 81 and an 84. (p. 1-34) Caution: none of this is true of Euro-metric tires (sizes without a P).

Choosing a tire and an inflation pressure has to satisfy two conditions, one for maximum load and one for "normal" load. The maximum load has to be <= the full tire capacity at the chosen pressure. The normal load has to be <= 88% of the tire capacity at the chosen pressure. (p. 1-03) That's the TRA requirement; there is a more generous 94% limit in the Federal safety standard FMVSS 110 S4.2.1.2.

We can get the maximum load straight from the Gross Axle Weight Rating on the door label (divided by two for per-tire load). The "normal" load comes from the curb weight of the car plus 3 150-lb occupants, two in front and one in back. (That's the specified "normal" occupancy for a 5-passenger car.)

The "max" numbers are easy to run:

	Column 1
0	[tr][td][/td][td]front[/td][td]rear[/td][td][/td][/tr]
1	[tr][td]gawr[/td][td]1970[/td][td]1685[/td][td]from door label[/td][/tr]
2	[tr][td]per tire[/td][td]985[/td][td]843[/td][td][/td][/tr]
3	[tr][td]capacity[/td][td]1019@35[/td][td]981@32[/td][td][/td][/tr]

So the "max" condition is easily met at 35/33, for either an 81 or an 84 tire. (I used capacity @ 32 from the table b/c it doesn't show 33.)

The "normal" numbers will take a bit more doing. Have to get the front and rear axle curb weights from the New Car Features Manual, then throw in 3 150-lb occupants.

I'm going to smush the occupants into one 450 lb blob on the centerline, 2/3 of the way from the rear to front seat. Eyeballing the underbody dimensions in the collision manual, this point looks to be about 110 cm forward of the rear axle, or 43% of the wheelbase, so this weight should distribute 194 lb to the front, 256 lb to the rear.

	Column 1	Column 2	Column 3
0	[tr][td][/td][td]front[/td][td]rear[/td][td][/td][/tr]
1	[tr][td]curb weight[/td][td]1700[/td][td]1065[/td][td]from New Car Features Manual[/td][/tr]
2	[tr][td]std occupants[/td][td]194[/td][td]256[/td][td]3 150-lb occupants	2 in front	1 in back[/td][/tr]
3	[tr][td][/td][td]----[/td][td]----[/td][td][/td][/tr]
4	[tr][td][/td][td]1894[/td][td]1321[/td][td][/td][/tr]
5	[tr][td]per tire[/td][td]947[/td][td]661[/td][td][/td][/tr]
6
7	[tr][td]capacity[/td][td]1019@35[/td][td]981@32[/td][td][/td][/tr]
8	[tr][td]x 0.88[/td][td]897[/td][td]863[/td][td]oops	we're 50 lbs over in front![/I] rear is ok.[/td][/tr]
9
10	[tr][td]x 0.94[/td][td]958[/td][td]922[/td][td]under the more generous FMVSS we're ok in front.[/td][/tr]

So because of the heavy front, no P175/65R14 tire can meet the TRA 88% normal load standard at 35/33, not even an XL tire. At 41 psi minimum in front, an XL (load 84) tire can meet this standard. A non-XL (load 81) cannot, at any inflation pressure.

Under the FMVSS 94% load limit, on the other hand, we're ok on either type of tire, even at the OEM 35/33. Obviously Toyota was able to get approval at those pressures, and tire vendors offer these tires for the application, so the FMVSS limit must be the one they have been held to.

So ... What I Learned Today (still for P-metric tires only):

• users of 81 or 84 load index P-metric tires are ok under FMVSS, at 35/33 or any higher pressure
• users of 81 index tires at higher pressures are still ok and probably getting better mileage, but no extra margin of load capacity
• users of 84 index tires at 35/33 are still ok, but not any more ok than the 81 folks
• drivers who use 84 index tires AND increase pressure to 41/39 or higher do have an extra margin of load capacity and meet the stricter 88% TRA figure.

-Chap

 

Inflation minimums for P-metrics and Euro-metrics

Putting it all together....

Here are the minimum inflation pressures for different P-metric and Euro-metric tires on a G1 Prius to meet the normal load US federal requirement (no more than 94% of tire capacity) or the more conservative TRA target (no more than 88%).

As explained here, P-metric inflation tables depend on the tire size and load index number only; Euro-metric tables depend on the load index number and whether the tire is XL or not, but not the size.

Keep scrolling ... vBulletin didn't put this huge gap ahead of tables, but xenForo does....

	Column 1
0	[tr][td][/td][td]for 94% normal-load limit[/td][td]for 88% normal-load target[/td][/tr]
1	[tr][td]capacity req'd (lbs)[/td][td]1008[/td][td]1077[/td][/tr]
2	[tr][td]P-metrics[/td][td][/td][td][/td][/tr]
3	[tr][td]P175/65R14 81[/td][td]35 psi[/td][td]-[/td][/tr]
4	[tr][td]P175/65R14 84 (XL)[/td][td]35 psi[/td][td]41 psi[/td][/tr]
5	[tr][td]non-XL Euro-metrics[/td][td][/td][td][/td][/tr]
6	[tr][td]81[/td][td]36 psi[/td][td]-[/td][/tr]
7	[tr][td]82[/td][td]35 psi[/td][td]-[/td][/tr]
8	[tr][td]83[/td][td]34 psi[/td][td]-[/td][/tr]
9	[tr][td]84[/td][td]33 psi[/td][td]36 psi[/td][/tr]
10	[tr][td]XL Euro-metrics[/td][td][/td][td][/td][/tr]
11	[tr][td]81 XL[/td][td]42 psi[/td][td]-[/td][/tr]
12	[tr][td]82 XL[/td][td]41 psi[/td][td]-[/td][/tr]
13	[tr][td]83 XL[/td][td]39 psi[/td][td]-[/td][/tr]
14	[tr][td]84 XL[/td][td]38 psi[/td][td]42 psi[/td][/tr]
15	[tr][td]85 XL[/td][td]37 psi[/td][td]40 psi[/td][/tr]
16	[tr][td]86 XL[/td][td]36 psi[/td][td]39 psi[/td][/tr]
17	[tr][td]87 XL[/td][td]34 psi[/td][td]37 psi[/td][/tr]

There are adjustments for high-speed driving. For example, an S tire is rated for 112 mph, but only with the minimum inflation increased by 3 psi for speeds above 99 mph. (TRA yearbook p. 1-06).

-Chap

 

Hi Bob,

I believe so - rims are to be marked for a max pressure, and mine, Toyota 14 x 5 1/2 JJ, are marked 45. Edit: I was mistaken on this point and set straight by tochatihu below. The FMVSS really does have a regulation requiring pressure markings, but only on non-passenger-car, non-one-piece rims. It doesn't apply to us, and the 45 on a Prius rim means something unrelated.

I don't think I found anything that can be simplified quite that far. If I were trying to summarize very quickly, I'd want to make the following points in order of importance:

1. You can'tsummarize the minimum PSI into a single number, you need to know:
   • P-metric or Euro-metric tire? (does size start with a P?)
   • Load index?
   • Tire size? For P-metric only. For Euro-metric the same load-index means the same capacity for any size, as long as you know:
   • SL or XL? For Euro-metric only; for P-metrics, knowing the size and load index tells you whether it's SL or XL.
2. Once you know which table to look in, you can find the inflation pressure to give you the needed capacity.
3. The hardest requirement to meet is the "normal load" limit for the front axle, so that's the one to pay attention to. The "normal" (figured by me, using the official recipe) front-axle load per tire is about 947 pounds.
4. The (US) Federal Motor Vehicle Safety Standard says this normal load should not be more than 94% of tire capacity. So, you must find a tire and pressure combo that will carry no less than 1008 pounds for the front (947 / 0.94). That's not hard; for example, 30 of the 31 tires returned by TireRack for a 2001 Prius will meet this requirement when inflated to 35 psi front, and the remaining one (the 86 XL winter tire) will if inflated to 36 psi front.
5. The (US) Tire and Rim Association would prefer you to keep normal load at no more than 88% of tire capacity. For that you need a tire and pressure combo that will carry 1077 pounds (947 / 0.88). This criterion is hard to meet; the OEM tires don't, at the OEM pressure.
   
   If you want to follow this more conservative guideline, far fewer choices are available without changing tire size: only 3 of the 31 listings on TireRack can possibly do it, and only when inflated to 41 psi for the OEM Potenza or the Dunlop SP10, or 39 psi for that 86 XL Blizzak.

-Chap

 

Hmm ... ok, thanks. That may mean I don't know where it is either - I did not find any intelligible markings on the inboard side of the wheel (and of course I haven't had an opportunity to see the surfaces enclosed by the tire, which would be a really inconsiderate place to put any markings).

Maybe there just is no pressure marking, because as I now look again at FMVSS 110 S4.2.2 I see that it strictly requires the information only for non-passenger-car wheels and even there it's optional for one-piece wheels.

Thanks for catching me there.

-Chap

 

Re: The dope on load/inflation minimums for P-metrics

I guess I never followed this up with one more tidbit I learned later.

The one question that had really bugged me, and that I was really trying to answer after looking all this up, was "ok, if the load capacities for 81 and 84 at the Toyota-spec'd pressures are exactly the same, then why on earth did Toyota spec the 84 tires?"

It turns out there could be an answer that doesn't have to do with load capacities. There's a preference for XL tires among drivers of tall, narrow vehicles. Apparently the extra plies also give more sidewall stiffness to resist sideways forces in a tall car. I found this by Google accident in a forum for classic VW Bus enthusiasts, who must know a thing or two about tall and narrow. And the Prius is kind of tall and narrow for a passenger car, so maybe that's what Toyota was going for.

Now, I put 81s on my own car after looking this all up, and in about 9000 miles I haven't had anything to complain about with lateral stiffness. But maybe I would feel differently if I were a more demanding driver, or if I lived in a gusty-wind area. I don't know.

-Chap