G1 tire requirements

[ChapmanF]

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

[tochatihu]

We usta bat this around quite a bit in the early yahoo groups. There are strongly held opinions that an 84 load is required. Also that 81 load tires can do the job if inflated and monitored well.

You are quite right that it is hard to load a Prius in a way that greatly increases the front wheels' loading. Pull the pax seat and put a load of bricks there will do it.

After the OE tires on my 2001 wore down, I used 175-width Goodyear Allegra, an example of the 'overloaded' tires. They worked very well at inflations near their sidewall max. When those wore down I went to Nokian i3 185-width XL tires. They cornered better, reduced the city mpg, but this effect was harder to see at highway speeds. When you go fast the fuel goes to aerodynamic resistance, and a smaller fraction to rolling resistance.

Another reason I moved up a size was so that I could run them soft for off-road driving. Few Prius owners would choose a tire with that in mind.

Going wider with 185s certainly removes the overload issue (whether or not it is a biggee). Often they will corner better, and therefore we might presume would also stop shorter under emergency braking. If you can live with a few less mpg, this seems an easy choice. If you ocassionally drive the Prius with very heavy loads (I did that too), wider tires make sense.

If fuel efficiency tops your list, you will probably do better with 175s, kept nicely inflated.

[gippah]

Wouldn't using a tire that isn't at its maximum load at its recommended PSI provide better driving comfort? Maybe that's the entire "mystery" here, Toyota just didn't want the Prius to have the feel of a shitty econobox.

[FHariton]

I think the problem we should be addressing is that the only XL tire that has always been available is a Bridgestone with a wear rating of 180. Properly inflated it is a 20,000 tire. This is something I have not had to deal with since the 1970's I note that my dealer once replaced my tires with non-XL tires (for the Echo) and those also lasted about 20K and I noticed no difference. Dunlop SP10's, although they had a slightly higher wear rating did not last longer and I am told are not available any longer. I can not feel as environmentally responsible as I would like as I add to the tire heap at such a fast rate.

[ChapmanF]

Originally Posted by gippah

Wouldn't using a tire that isn't at its maximum load at its recommended PSI provide better driving comfort? Maybe that's the entire "mystery" here, Toyota just didn't want the Prius to have the feel of a shitty econobox.

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

[ChapmanF]

Originally Posted by FHariton

I think the problem we should be addressing is that the only XL tire that has always been available is a Bridgestone with a wear rating of 180. Properly inflated it is a 20,000 tire.

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

[ChapmanF]

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

[mikel 52]

I recently put Bridgestone B-381 P185-65-14s on my 2002. They are LRR tires. OE on civic hybrids. Some people claim to have 75000 on them. They are definitely better than the Michelin Pilots that were on it when I bought it. I saw a list some where that had their rolling resistance shownas something liek .0067.

Mike

[ChapmanF]

Originally Posted by mikel 52

I saw a list some where that had their rolling resistance shownas something liek .0067.

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

[nitewolfgtr]

I just installed 4 Falken Ziex ZE912 195/60R14 on my '01 Prius and love them. Lost approx. 2 MPG but much better handling!