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#1

Posted 15 April 2006 - 12:31 AM

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I believe most people are not aware of the relationship between air pressure and load index when replacing their tires. Me too...


source: http://www.tirerevie...pe=art&id=5278

Double Jeopardy
by Scott Blair

In March 2006 / In Focus: Performance

When you sell a set of non-OE tires, do you set the air pressure the same as it was in the OE tires?

Do your sales staff, manager and technicians understand the importance of load index? Can they read a load and inflation table? Can they recite, from memory, the inflation rule – that the replacement tire must meet or exceed the OE tire’s load carrying capacity?

If the answer’s “no,” you might be taking a big liability risk. Not only do you have to train your staff in these matters, you also have to document that training to be fully protected against potential legal losses.

Concerned? You should be. In fact, sit down and hold on; we’re going to hit you right between the eyes. We’re going to show you how you can sell the right tire size but the wrong load index.

More Complex Than Ever

Times have changed. Five years ago, a specific tire size had only one load index number. Not today. Now, a given tire size may have as many as five different load index numbers molded on the sidewall, making it impossible just to look at the size of a given tire and make the assumption that the load index number is the same just because the tire size is same. These days, you have to look at each tire, along with its complete sizing information, before you can make any fitment decisions.

The following case studies will illustrate the importance of the relationship between load index and tire size.

Case File: One

A customer wants a set of 20-inch tires on his 2003 Lexus LS430. The OE tire is a P225/55R17 95H, standard load (SL) with a load index of 95 and recommended cold inflation pressure of 32 psi front/rear. The overall diameter (OD) is 26.8.

You have a stack of P255/35R20 93Hs on the floor with an OD of 27, so you sell him a package deal. “The OD is correct, so it must the right tire,” you think to yourself. Not so fast. Take a look at the load index of that 20-inch tire. Notice that it is 93. The OE tire’s load index was 95. Install that tire, and you just might open the door to a liability suit if that customer gets into a wreck after leaving your shop.

“How can this be?” you ask. I’ll show you: The OE P225/55R17 95H tire inflated to the recommended air pressure equates to 1,477 pounds of load capacity. A standard load (SL) P-metric tire reaches its maximum load capacity at 35 psi, regardless of the maximum inflation pressure listed on the sidewall!

Looking at the load and inflation table, we see that the load index of 93 on the P255/35R20 93H tire equates to only 1,433 pounds of load capacity at 35 psi. It’s only 44 pounds per tire. “That’s not going to matter,” you argue. Tell that to the judge when he reads a settlement against you for some ridiculous amount of money because “the replacement tire did not meet or exceed the OE tire’s load carrying capacity.”

The correct tire for this plus-size application should be a euro-metric (There is a difference!) 255/35R20 97H XL (extra load) tire, which has a load index of 97. Let’s say you installed this tire, which we now know is correct, but you used the OE air pressure of 32 psi. At 32 psi, this tire will only carry 1,290 pounds.

If you do the math, you’ll see the following: 1,477-1,290=187 pounds overloaded per tire or 748 pounds totally overloaded. The correct cold inflation pressure should be adjusted to 38 psi front/rear to equal the 1,477 pounds of load capacity specified by the vehicle manufacturer.

Case File: Two

A customer comes in and wants a set of 20-inch tires on a 2004 Dodge Ram 1500 SLT Crew Cab RWD truck. The OE tire is a P265/70R17 113R (SL), with a recommended cold inflation pressure of 35 psi front/rear. You have a set of take-offs from another Ram 1500 already mounted with P275/55R20 111S tires. Since they came off of a Ram 1500, they should fit your customer’s truck, right? Sure, they might ‘fit,’ but they won’t be the right tires.

The OE tire inflated to 35 psi equates to 2,535 pounds of load capacity. Take a look at the 20-inch tire’s load index: 111 SL. As mentioned earlier, a (SL) P-metric tire reaches its maximum load capacity at 35 psi. Therefore, a lower load index will decrease the maximum load capacity. If you were to install this tire and inflate it to 35 psi, the tire would be overloaded by 132 (2,535-2,403) pounds per tire or 528 pounds totally overloaded.

The correct tire would be the optional tire that Dodge uses on its heavier trucks – P275/60R20 114S (SL). The load and inflation table reveals that, with the higher load index, we can adjust air pressure to 33 psi and still meet the OE recommended cold inflation pressure.

In this case, however, you should still inflate to 35 psi, which exceeds the minimum recommended load capacity. The load capacity numbers were not technically correct. Can you guess why?

Notice that, in Case One, a P-metric tire was installed on a light truck. According to the RMA, the load capacity of a P-metric or euro-metric tire installed on a light truck has to be reduced. Divide the load capacity of the P-metric or euro-metric tire by 1.1 to determine the load capacity for a light truck application.

A common example would be a P235/75R15 105S (SL) tire inflated to 35 psi. The load capacity for a passenger car application would be 2,028 pounds. Placing this same tire on a light truck equates to 1,844 (20281.1) pounds of load capacity. If this tire was the OE size for the truck, this calculation would already be done and reflected on the door placard.

There are other factors involved when installing LT-metric or flotation tires on a light truck that has OE P-metric or euro-metric tires, so make sure that you receive proper instruction before making these types of changes.

New Legislation

Finally, you should be aware of some new legislation regarding the installation of aftermarket wheels and non-OE tires on a new vehicle that hasn’t been titled yet. The new rule, Federal Motor Vehicle Safety Standard #110, which took effect Sept. 1, 2005, states that if you “alter” a new vehicle by installing different tires or wheels, you must place a new tire inflation and load capacity placard over the original to indicate the new front/rear/spare air pressures to the buyer.

SEMA has compiled recommendations to help you avoid any problems that may arise due to differing interpretations of the ruling. You can order the placards from various label makers.

If you are interested in getting this information, feel free to contact me, and I will direct you to several companies compiled by NHTSA. This standard applies to changes in the vehicle’s weight capacity, as well. If you install products that add weight to the vehicle, you will need to indicate the new weight capacity of the vehicle, which will be the manufacturer’s listed weight less the amount you added. NHTSA does allow for a 0.5% deviation of the gross vehicle weight rating without replacing the placard, but this may change in the coming months.
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#2

Posted 15 April 2006 - 01:49 PM

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Wow!! Very impressive... but I'm lost.... so technical.... sweatdrop.gif
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#3

Posted 15 April 2006 - 01:54 PM

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agree, can someone translate into england ?
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#4

Posted 17 April 2006 - 11:11 AM

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lost too!
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#5

Posted 17 April 2006 - 11:25 AM

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The England is simple. Non-stock tire sizes = different tire pressure. That is what I've been warning everyone here about. 2psi is enough to make your car behave very differently. Cannot use back stock air pressure.
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Men are like oysters, closed to suffering.
http://www.spritmoni...ail/264162.html

#6

Posted 17 April 2006 - 11:52 AM

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anyone's england powderful enough ? laugh.gif
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#7

Posted 17 April 2006 - 12:21 PM

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Good read.

Summary: For case one, your tires must be able to support the load

For case two, ditto above + safety factor for the light weight vehicle to get the correct tyres.
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#8

Posted 17 April 2006 - 12:57 PM

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If not mistaken, he's referring to load index which many failed to notice when upsizing cos nowadays a tire size could have multiple load indexes.

Always change tires with the same load index or higher.
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#9

Posted 17 April 2006 - 01:07 PM

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Other than that, you can always compensate with the tire pressure. A lot of things to play around.

A lot of people dunno. Use different load index, use same tire pressure as stock load index.
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#10

Posted 17 April 2006 - 01:13 PM

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This a the other part which I'm still confused or ignorant. Please enlighten me. Tks!

Eg. Original size is 175/65R14 82H -> new size is 185/55R15 84V or even 195/50R15 86H (sorie its imaginary)

#11

Posted 17 April 2006 - 01:19 PM

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As you upsize, the profile gets thinner if you have done it correctly. There isn't much sidewall to support the load. So you need higher pressure usually. That is why upsized tires are uncomfortable. They are rock solid hard!

So theoretically, for every inch increase in size, you add 2psi more. For the heavy end, you need to add 4psi more.

For the 55 profile compared with the 50 profile, you can get away with slightly less tire pressure. A lot of things to play around with.
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#12

Posted 17 April 2006 - 01:30 PM

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Then as we discussed in other topic, I guess my car manufacturer a bit when its stated on the car n manual that a +1 size from 175/65R14 to 185/55R15 still pump the same pressure.

So if the max required is 33psi then by pumping 35psi it should be correct n not 33psi as stated? However at 34psi I oredi feel a better pickup n FC

#13

Posted 17 April 2006 - 02:08 PM

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This problem occurs not just with consumers, in fact alot of tyre shops do not have this knowledge.

I think is up to individual to understand and apply the knowledge. If not just stick car manufacturer's recommendation.

Cheers
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#14

Posted 17 April 2006 - 05:57 PM

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But in the end, it's still dependent on the load your car is carrying.
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#15

Posted 17 April 2006 - 09:40 PM

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I'm not a guru in this subject, just that it affected me. So, here's the story in layman terms for the benefit of everyone... maybe one day it may save someone angel.gif ... haha.. good luck, be safe!



DISCLAIMER: The information here is what I have learnt and I may not be 100% correct. I am not a certified tire specialist or a rubber specialist lipsrsealed.gif. I'm not responsible for any accident, injuries or loss of life as a result of using the information provided here. Take my entry here with a pinch of salt. It is recommend that you understand the concept behind it so that you know what you are applying.

DISCLAIMER: This article is not 100% applicable to SUV/commercial vehicle


1. Background
- when designing a car, the manufacturer will choose an appropriate tire to support its weight
- some possible factors
. - vehicle weight
. - potential load
. - handling characteristic
. - vehicle performance (remember that at high speed, the down force will add some substantial weight)

thumbsup.gif Take it that there are good reasons for the manufacturer to select a certain load rating


2. Industrial standards
- to make it easy for everyone, a standard is created
- unfortunately, there are 2 types of standards for passenger cars
. - p-metric & euro metric (it must be the Americans!)
. - p-metric tires are marked as P215/45R17 91V (note the P)
. - euro metric tires are marked as 215/45/R17 91V (without the P)
- we are using euro metric tires
- why we need to take note of p-metric and euro-metric? because the max load is measured at different inflation
- the standards also defined extended load tire which is measured at a higher inflation

- if you visit U.S. websites, the example may be using p-metric tires

thumbsup.gif so, max load is at 2.5 bar (250kpa) regardless of max inflation which the tire can handle



3. Tire - Manufacturer's Spec

- for simplicity,
. - we will assume that our vehicle will not exceed 160km/h
. - we will not consider extended load (XL) tire where the max load is at higher inflation

- tire size: 205/50R17 89V
. - max load is 580kg 2.5bar (see table below)

- manufacturer recommended inflation, normal 2.2bar F/R (32psi) = 525kg
. - total = 525kg x 4 = 2100kg

- manufacturer recommended inflation, full load 2.3bar/3.1bar = 545/580kg
. - total = 545kg x 2 + 580kg x 2 = 2250kg



4. Tire - Upgrade, Plus+2
- say you are currently using the original 15" wheel and decided to upgrade to 17"
- still 17" but deviate from the manufacturer's recommendation
- tire selected : 215/45R17 87W
. - max load is 545kg 2.5bar (see table below)

- manufacturer recommended inflation, normal 2.2bar (32psi) = 525kg
. - total = 490kg x 4 = 1960kg
. - so with this tire, we are short of 2100-1960 = 140kg (=> overload!)
. - at normal load, to support the same amount of weight (525kg), we need to increase the inflation to 2.4bar (240kpa)

- manufacturer recommended inflation, full load 2.3bar/3.1bar = 510/545kg
. - total = 510kg x 2 + 545kg x 2 = 2110kg
. - so with this tire, we are short of 2250-2110 = 140kg (=> overload!)
. - at full load, to support the same amount of weight (545/580kg), we need to increase the inflation to 2.5bar (250kpa) BUT the rear max out at 545kg regardless of inflation in excess of 2.5bar (250kpa)
. - so after final adjustment, we have
. 545kg x 2 + 545kg x 2 = 2180kg
. 2250 - 2180 kg = 70kg short (=> ie the rear tires will be overloaded!)


5. Summary
thumbsup.gif never choose a tire of lower Load Index
thumbsup.gif if your replacement tire has the same Load Index, then same inflation pressure
thumbsup.gif if your replacement tire has higher Load Index, you may use the same inflation pressure or adjust base on the inflation pressure load capacity chart
thumbsup.gif never go below 2bar
thumbsup.gif Load Index has nothing to do with the speed rating (ie. H,V,W,Y,Z,ZR...)
thumbsup.gif bear in mind that with the adjustment, the tire with lower load rating may experience high load stress than one that is correctly specified.

thumbsdown.gif calculation may not be application to tire with puncture repair
thumbsdown.gif calculation may not be application to worn out tire




CODE


Load Index (LI) vs Inflation (bar, 1bar=100kpa), table values in kg

  2.0 2.1 2.2 2.3 2.4 2.5 (bar)

80 375 390 405 420 435 450

81 385 400 415 430 445 462

82 395 415 430 445 460 475

83 405 425 440 455 470 487

84 420 435 450 470 485 500

85 430 450 465 480 500 515

86 445 460 480 495 515 530

87 455 475 490 510 525 545

88 470 485 505 525 540 560

89 485 505 525 545 560 580

90 500 520 540 560 580 600

91 515 535 555 575 595 615

92 525 550 570 590 610 630

93 545 565 585 610 630 650

94 560 585 605 625 650 670

95 575 600 625 645 670 690

96 595 620 640 665 685 710

97 610 635 660 685 705 730

98 625 650 675 700 725 750

99 650 675 700 725 750 775

Table obtained from http://www.generalti...res_2005_en.pdf
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#16

Posted 18 April 2006 - 08:42 AM

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Tks 4 the detailed/simplier explanation. Pardon my poor knowledge in understanding the earlier extract.

#17

Posted 06 September 2006 - 09:37 PM

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Now we get the picture thumbsup.gif
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#18

Posted 23 December 2007 - 03:53 PM

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Most aftermarket tires have more or around the same load index.

If you are changing back to the same size tires, it better that one pays attention to the load index. More nvm...but not lesser. If you are upgrading to those plus 1 or 2 option, the load index of the tires is usually higher.

For those who buys 2nd hand cars....just watch out for the tires on the car. It may look new and nice...but check if its the correct size. When I got my 2nd hand Picnic....the previous owner or the dealer used a undersize tire for me...stock size is 205/55/15 but he used a 195/55/15 cos the latter size is widely avaliable and cheaper.
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I got no signature ??

#19

Posted 12 June 2008 - 11:43 PM

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thanks for sharing
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#20

Posted 21 June 2008 - 12:06 AM

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just MHO
for passenger car i think is OK tohave 1 or 2 mark below or lower than original because by working out the weight the tyre can tahan see below
LI 86 445kg x 4 tyre = 1780kg
LI 84 420kg x 4 tyre = 1680kg
so the diff is 100kg in total
if your car curb weight is 1200kg so you still have room for 1680-1200=480kg pay load
so divided by average 70kg per pax you still can carry 6pax x 70kg plus 60kg lagguage.
BUT for MPV you got to be very careful.
this MPV could carry up to 9pax+ lagguage,so you got be very careful when you up size your tyre,1 more very important item to know is your sport rim,my advice is try not to go for those light weight rim,some of them are more suitabel for saloon car
for example 9pax x 70kg is = 630kg + 70kg = 700kg plus your MPV weight 1600kg
you better make sure your tyre and rim are strong enough to carry total weight of
2300kg (always remember to go for strong rim for MPV)
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