G1 tire requirements

[ChapmanF]

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.

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:

	front	rear
gawr	1970	1685	from door label
per tire	985	843
capacity	1019@35	981@32

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.

	front	rear
curb weight	1700	1065	from New Car Features Manual
std occupants	194	256	3 150-lb occupants, 2 in front, 1 in back
	----	----
	1894	1321
per tire	947	661
capacity	1019@35	981@32
x 0.88	897	863	oops, we're 50 lbs over in front! rear is ok.
x 0.94	958	922	under the more generous FMVSS we're ok in front.

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

[ChapmanF]

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.

	for 94% normal-load limit	for 88% normal-load target
capacity req'd (lbs)	1008	1077
P-metrics
P175/65R14 81	35 psi	-
P175/65R14 84 (XL)	35 psi	41 psi
non-XL Euro-metrics
81	36 psi	-
82	35 psi	-
83	34 psi	-
84	33 psi	36 psi
XL Euro-metrics
81 XL	42 psi	-
82 XL	41 psi	-
83 XL	39 psi	-
84 XL	38 psi	42 psi
85 XL	37 psi	40 psi
86 XL	36 psi	39 psi
87 XL	34 psi	37 psi

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

[bwilson4web]

Hi,

I want to make sure I summarized this correctly:

• standard rims - rated at 45 psi max
• 40 psi - minimum required to achieve 95% of load

Is that about right?

Excellent work although I'm curious about the "catalog" you mentioned.

Thanks,
Bob Wilson

[ChapmanF]

Hi Bob,

Originally Posted by bwilson4web

standard rims - rated at 45 psi max

I believe so - rims are to be marked for a max pressure, and mine, Toyota 14 x 5 1/2 JJ, are marked 45.

Originally Posted by bwilson4web

40 psi - minimum required to achieve 95% of load

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't summarize the minimum PSI into a single number, you need to know:
   1. P-metric or Euro-metric tire? (does size start with a P?)
   2. Load index?
   3. 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:
   4. 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

[tochatihu]

Slow down folks. 45 on the Prius wheel means 45 millimeters positive offset. If these wheels are in fact marked with a maximum psi then I do not know what (or where) it is. I would guess in the range of 90 psi because this is what I have heard/read for other aluminum alloy wheels.

[bwilson4web]

Originally Posted by tochatihu

Slow down folks. 45 on the Prius wheel means 45 millimeters positive offset. If these wheels are in fact marked with a maximum psi then I do not know what (or where) it is. I would guess in the range of 90 psi because this is what I have heard/read for other aluminum alloy wheels.

THANKS!

This was driving me nuts.

Bob Wilson

[ChapmanF]

Originally Posted by tochatihu

Slow down folks. 45 on the Prius wheel means 45 millimeters positive offset. If these wheels are in fact marked with a maximum psi then I do not know what (or where) it is.

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