Author Topic: tire pressure (Read 3785 times)

bushwalker · « **on:** November 05, 2006, 12:18:27 am »

Hi,I've got a 1998 gmc half ton 4/4.I put some 10 ply tires on it for off roading and some gravel road driving.My question is what is minimum tire pressure I can savely have in them (I don't carry any weight in the box )? I phoned 4 different tire stores and they all give a different answer.The truck came with P245R75/16. Thanks.

bushwalker.. tires on the truck now are LT235R85/16..

morty · « **Reply #1 on:** November 05, 2006, 01:57:37 am »

Typically the P245/75/16 has a load index of 109. The 245/75/16 10ply should have a load index of 120. Rule is for every +1 in load index you -1 in pressure. So, theoretically you could lower the pressure 11psi. However, this would be far too low to keep the tire on the wheel. I would probably run the tire at a minimum of 40psi. If still to firm you could go lower, but you might start to get some increased shoulder wear.

HeliDriver · « **Reply #2 on:** November 05, 2006, 03:31:13 am »

Quote from: morty on November 05, 2006, 01:57:37 am

Typically the P245/75/16 has a load index of 109. The 245/75/16 10ply should have a load index of 120. Rule is for every +1 in load index you -1 in pressure. So, theoretically you could lower the pressure 11psi. However, this would be far too low to keep the tire on the wheel. I would probably run the tire at a minimum of 40psi. If still to firm you could go lower, but you might start to get some increased shoulder wear.

I thought that the rules went out the window when comparing P-metric tires to LT tires.

Even though the LT tire has a higher load index, isn't that load index also based upon a much higher pressure than that of a passenger car tire? (50 psi vs. 35)

I would have thought to add 10 or 15 psi to GM's recommended pressure, not subtract it.

capriracer · « **Reply #3 on:** November 05, 2006, 07:42:33 am »

Bushwalker,

You just had a demonstration of why you got 4 different answers. Even Morty - who is quite knowledgeable in this area - got it wrong.

Your vehicle came with P245/75R16's inflated to 35 psi. The load carrying capacity of the tire at that pressure is 2064 # (hint: For Light truck applications, Passenger car tires are derated 10%)

For an LT235/85R16 to carry 2064 #, the pressure has to be 46 psi.

Hope this helps.

tpl · « **Reply #4 on:** November 05, 2006, 10:07:04 am »

That answer may help but it doesn't answer the question.

A 1/2 ton truck weighs, say 2 tons and has a load carrying capability of 1/2 ton so thats a total of about 5500 lbs. thats 1400 lb/s tire or there abouts. call it 1600 and therefore if the LT235/85R16 carries 2064lbs at 46 psi it should safely carry 1600 lbs at some lower pressure... I dont know what but presumably somewhere between 35psi and 46 psi.

HeliDriver · « **Reply #5 on:** November 05, 2006, 01:41:23 pm »

Quote from: tpl on November 05, 2006, 10:07:04 am

That answer may help but it doesn't answer the question.

A 1/2 ton truck weighs, say 2 tons and has a load carrying capability of 1/2 ton so thats a total of about 5500 lbs. thats 1400 lb/s tire or there abouts. call it 1600 and therefore if the LT235/85R16 carries 2064lbs at 46 psi it should safely carry 1600 lbs at some lower pressure... I dont know what but presumably somewhere between 35psi and 46 psi.

Maybe so, but using that logic it then follows that it would be OK to inflate the stock tires to only 25 psi (which would also give a load limit of around 1600 lbs).

Using a pressure (or load limit) 30% lower than what the manufacturer has recommended doesn't seem like a good idea, IMO.

tpl · « **Reply #6 on:** November 05, 2006, 02:43:46 pm »

I agree with your logic but I dont agree with the numbers ... but I may be missing something...

I am presuming that the statement "Your vehicle came with P245/75R16's inflated to 35 psi." means that if the truck has its 1/2 ton rated load that is the tire pressure. Capiracer then says that the currently fitted tires need 46 psi for that same load

Bushwalker says that he does not carry a load in the box so the truck is essentially running unloaded ( Lets ignore the weight of Bushwalker here). SO he can safely run his current tires at less than 46 PSI. For sure he should not run them at 25 psi but maybe he could run them at 35 psi.

Now I dont bknow what this means "hint: For Light truck applications, Passenger car tires are derated 10%)
" does it mean that those P type tyres could run 2046 lbs + 10% on a car? i.e. 2250 lbs? or that they should not be loaded past 1850 lbs on a truck ?

HeliDriver · « **Reply #7 on:** November 05, 2006, 03:12:22 pm »

I guess that a lot of it boils down to how much of a safety margin the manufacturer has engineered in.

Maybe things are different for trucks (since they can vary so much in weight when fully loaded vs. unloaded), but I've never heard of anyone suggesting that it's acceptable to go lower than the vehicle manufacturer's recommended tire pressure settings.

If GM's tire placard says "inflate to 35 psi", then I'd be inclined to do as it says. If the placard were to say "inflate to 35 psi when fully loaded; inflate to 25 psi when vehicle is empty", that would be a different story.

(BTW, the above example is for the stock tires. If GM had fitted the truck with LT tires from the factory, the placard would read 46 psi instead. Same end result as far as load carrying, though.)

Quote from: tpl on November 05, 2006, 02:43:46 pm

Now I dont bknow what this means "hint: For Light truck applications, Passenger car tires are derated 10%)
" does it mean that those P type tyres could run 2046 lbs + 10% on a car? i.e. 2250 lbs? or that they should not be loaded past 1850 lbs on a truck ?

I believe that this means your first suggestion: i.e. the same tires would be allowed to carry 2250 lbs if they were on a car.

morty · « **Reply #8 on:** November 05, 2006, 07:57:15 pm »

I based my answer on LT245/75/16 LRE and simply on load index. My understanding is that if the load index is increased then you can lower the pressure and carry the same weight. The LT245/75/16 LRE has an index of 120 and the P245/75/16 a load index of 109.

morty · « **Reply #9 on:** November 05, 2006, 11:48:53 pm »

I am going to dig a little deeper on this and come up with a concise answer. I would like to have a definitive answer myself. We dont do a lot of conversions from P to LT, so I would like to brush up on this subject. Be back soon...

bushwalker · « **Reply #10 on:** November 05, 2006, 11:50:14 pm »

Thanks everyone who gave an answer,still confused about the pressure so I will continue with the pressure I now have in the tires which is 50 psi.

bushwalker

Trainman · « **Reply #11 on:** November 06, 2006, 01:28:40 am »

On my Pathfinder, I went from P rated to LT rated and upped the psi from 26 to 40.

Note my P rated tire was good to a maximum of 38 psi, while the LT tire is good to 80 psi. 40 seemed a good compromise for ride, wear and handling and was recommended by the tire shop (they do a lot of P to LT tire conversions due to the nature of the vehicles in use here and what they are used for). 40 has given good, even wear across the width of the tire.

I await morty's findings with bated breath

capriracer · « **Reply #12 on:** November 06, 2006, 08:32:16 am »

It's obvious I need to explain where my numbers came from.

A P245/75R16 has a rated load of 2271 # at 35 psi. This is called a 109 Load Index. BTW, this is a P metric tire and the Load Index isn't quite accurate for the load carrying capacity - a quirk since P metric tires are a US based tire standard and Load Indices are a European kind of thing.)

Now, if the tire is used in a passenger car application (For this size, it's obsurd, but for other sizes it isn't!), then the MAXIMUM load that can EVER be applied to this tire is 2271 #. As has been pointed out, it is good engineering practice to have some reserve capacity - my college professior used to say "overdesign / underutilize".

For Light Truck applications, Passenger car tires are supposed to be derated 10%. I'm not sure exactly why that is, but I suspect it has to do with the stiffer suspensions of trucks and the likelihood of being loaded beyond the load capacity of the tire (Those pickup beds are tempting targets for heavy loads.)

That means that the MAXIMUM load a P245/75R16 can EVER carry when on a pickup is 2064 #. This is how GM designed it, and I'm not in a position to rethink what GM engineers spent a lot of time fretting over. Remember, they put a label on the tire that tells what the pressure is supposed to be and there are plenty of sharp lawyers out there who can second guess this decision at their leisure.

For an LT235/85R16 LR E it has a load carrying capacity of 3042 # at 80 psi. It is the same for an LT245/75R16 LR E. BTW this is 120 Load Index.

The key is that this load capacity (Load Index, too!) is the inflation pressure this occurs at - Morty, this means you can't apply the usual rules you use for passenger car tires about Load Index and pressure. BTW, this is also a problem when changing between Standard Load (SL) and Extra Load (XL) passenger car tires.

So, to get the same load carrying capacity in a LT235/85R16, you need 46 psi. Turns out that there is an option where this truck comes with LT245/75R16's and GM says to use 45 psi.

Now we can argue about the truck being empty, but consider that the front tires hardly ever see a different load - the load in the bed would be centered over the rear axle. And GM does test their vehicle both empty and fully loaded and have designed the springs, shocks and sway bars such that the vehicle has predictable handling in both conditions. Again, I'm not in a position to second guess what GM engineers spent a lot of time workigm on and deciding.

tpl · « **Reply #13 on:** November 06, 2006, 08:46:29 am »

Thank you Capiracer. Now I understand and see exactly the issue. 46 psi is it then. But why are LT tires designed to have such high inflation pressures? To keep them reasonably narrow?

articsteve · « **Reply #14 on:** November 06, 2006, 12:46:59 pm »

I had a 3/4 ton Silverado 4/4 diesel with Khumos rated to 80 psi. I kept them at 44 # and ran a lot of gravel and snow and got great wear.

PS. Once you own a well used GMC truck and you repair it yourself you soon come to realize nothing was "engineered".

capriracer · « **Reply #15 on:** November 06, 2006, 02:56:59 pm »

Quote from: tpl on November 06, 2006, 08:46:29 am

.....But why are LT tires designed to have such high inflation pressures?..............

LT tires are the little brothers of Over The Road Truck tires and in many respects are designed the same way, which is to say, with high loads and inflation pressure compared to how large the tires are. This concentrated loading creates its own problems in design, but I don't need to do into that.

However, one of the odd things is when we compare a P metric tire with an LT that has the same dimensions - example P245/75R16 vs LT245/75R16 - the LT requires 10 psi more to get the same load carrying capacity. Why?

I can't really answer the question because the situation predates my involvement, but of all the people that visit this forum, I'm in the best position to give you some insight. So here goes:

Many, many years ago, Passenger Cars and Light Trucks were clearly different and the tires used reflected that difference. Passenger car tires were small, low pressure tires and Light Truck tires were larger, high pressure tires. Based on what worked at the time, tire engineers developed formulas that were used to calculate the loads for a given size. Since the tire engineer's experiences were different with these 2 types of tires, the formulas were different, too!

Well.....Passenger cars got larger and trucks got smaller, and it was discovered that you could fit a large passenger car tire on a light truck and get a much softer ride. This trend continued and Passenger Car tires kept getting larger until they reached areas where only LT tires used to operate. Because the load formulas were different, there is this odd and peculiar load vs pressure thing that occurs in this cross over area.

But since LT metrics are designed and tested at the load and pressures specified, it is not a good idea to compare LT loads / pressures to P metric loads / pressures.

morty · « **Reply #16 on:** November 07, 2006, 02:59:33 am »

Yes I do feel I am, in err...I have pulled out our TIA certification manual and intend to study this further. They have some conversion tables and I am hoping to find some sort of formula that makes it easier for all/myself. Be back soon....as soon as possible...we are swamped and have not had an opp to really go into this.

morty · « **Reply #17 on:** November 09, 2006, 01:49:43 am »

Definitely in err...

OK...no formula, but have looked over "The Tire and Rim Association's Load & Inflation Tables". I am seeing that they show a load carrying of 2271 Lbs @ 35PSi, which is what capriracer has already mentioned. Then we must subtract 10% of the load carrying becasue the P-Metric tire is fitted to a light truck. (link with more info:http://www.nittotire.com/assets/safety/Replacing%20Tires%20on%20Light%20Trucks.pdf) This gives us the 2064lbs load carrying. According to the table you would be required to have approximately 45psi in a LT235/85R16 LRE to have the same load carrying capacity as the P245/75R16. The load carrying capacity of this tire at max inflation (80psi) is 3042 lbs.

Capriracer...how did you come up with your numbers? Did you use the same source or just use the 10psi rule?

The reason for having the higher inflation is to allow for more load carrying capacity. The higher the pressure and the larger the contact patch...the more weight the tire can carry.

Sometimes you just need to review those text books...it does get a little rusty without use....

capriracer · « **Reply #18 on:** November 09, 2006, 07:20:35 am »

Quote from: morty on November 09, 2006, 01:49:43 am

..............Capriracer...how did you come up with your numbers? ......

The same way you did.

BTW be careful because TRA, ETRTO, and JATMA have slightly different load tables - another one of those "quirks"

blur911 · « **Reply #19 on:** November 09, 2006, 10:13:12 am »

Hi Guys
I'm just wondering what the advantage is to run at lowest allowable pressure as was the original question? Wouldn't that mean more wear and worse fuel mileage? Is the only advantage ride comfort?

Generally my F-250 doesn't move unless it has a load, so I keep it at 60psi or so.