New LT Tire Pressure Recommendations

hutch08 · February 1, 2015

Just picked up my 2015 Sierra All-Terrain, had a 2.5 RC lift/level installed along with some 295/55R20 Toyo AT2 Extreme tires.
The shop set them at 32psi on all 4 corners, but I have a feeling that I should be running more air than that.

According to some calculations I found online,

Stock Goodyear SR-A 275/55r20 have a load rating of 111(2400lbs) at max 51psi but the door recommends 32psi.

The Toyo AT2 Xtreme 295/55r20 have a load rating of 123(3420lbs) at max 80psi.

Goodyear
2400lbs / 51psi = 47lbs/psi (max carrying capacity of each tire per psi)

Toyo
3420lbs / 80psi = 42.75lbs/psi (max carrying capacity of each tire per psi)

Recommended stock 32psi X 47lbs/psi = 1504lbs (actual load per tire)

1504lbs / 42.75lbs/psi = 35psi (new recommended psi)

I guess what all that mumbo jumbo means is to retain the same carrying capacity of the stock tires, I need to run 35psi in the Toyo's.

Anyone have real world experience with a similar sized/load tire? Just looking for the best overall ride quality, fuel economy and tread wear.

jadatis · February 1, 2015

Your stock Goodyear tires are probably P-tires with real maximum pressure of 51 psi, but pressure needed for maximum load up to 160km/99m/h ( also called maxloadpressure/reference-pressure/AT-pressure) of 35 psi for tires for american marked.

European market tyres use 36 psi as maximum pressure . googled the tires and found also XL/reinforced/Extraload with Loadindex 117.

Knowing this the calculation of original is next 2400lbs/35psi=68.57 lbs/psi x 32 psi= 2194 lbs a tire

Then the toyo's 2194lbs/42.75= 51.32 psi wich is al lot more then your 35 psi.

A stiffer tire, like the LT E-load is , needs more pressure for the same load .

I registered to this forum to give you this answer, live in Holland , and call myself tire-pressure specialist nowadays.

The calculation the tire- and car-makers use is different in Europe and America, and has a curve in the line if you make graphics of it.

The formula has a power in it.

But you can use an "on the road methode and that asumes a part of the load been carried by the construction of the tire , and rest is carried by the surface on ground times pressure .

I stated this construction load at 45 kg/100 lbs.

Will give a picture of different calculations and my "on the road methode"

hutch08 · February 1, 2015

Your stock Goodyear tires are probably P-tires with real maximum pressure of 51 psi, but pressure needed for maximum load up to 160km/99m/h ( also called maxloadpressure/reference-pressure/AT-pressure) of 35 psi for tires for american marked.

European market tyres use 36 psi as maximum pressure . googled the tires and found also XL/reinforced/Extraload with Loadindex 117.

Knowing this the calculation of original is next 2400lbs/35psi=68.57 lbs/psi x 32 psi= 2194 lbs a tire

Then the toyo's 2194lbs/42.75= 51.32 psi wich is al lot more then your 35 psi.

A stiffer tire, like the LT E-load is , needs more pressure for the same load .

I registered to this forum to give you this answer, live in Holland , and call myself tire-pressure specialist nowadays.

The calculation the tire- and car-makers use is different in Europe and America, and has a curve in the line if you make graphics of it.

The formula has a power in it.

But you can use an "on the road methode and that asumes a part of the load been carried by the construction of the tire , and rest is carried by the surface on ground times pressure .

I stated this construction load at 45 kg/100 lbs.

Will give a picture of different calculations and my "on the road method"

Holy! That's way more in depth than what I "figured out"..

So in simple terms, you're saying I would need approximately 50psi in the Toyo's to match the load capacity of 32psi in the stock Goodyears?

Appreciate the help.

newdude · February 2, 2015

Holy! That's way more in depth than what I "figured out"..

So in simple terms, you're saying I would need approximately 50psi in the Toyo's to match the load capacity of 32psi in the stock Goodyears?

Appreciate the help.

Yep. http://toyotires.com/sites/default/files/page-files/TSD-12-011%20Replacing%20Tires%20on%20Light%20Trucks.pdf

Found this somewhere on this site, so putting it here as well. An LT will have run more pressure to obtain the same load rating as the P tire.

jadatis · February 2, 2015

And this was only the beginning .

If you would use my off the road methode wich comes close to the ever to be constructed ideal formula, the calculation would be like this.

P-tires 2400-100= 2300/35=65.7lbs/psi x 32=2103 lbs+ 100= 2203 lbs

For the E-load tires for wich AT pressure is given on sidewall

3420lbs-100 lbs= 3320 lbs / 80 psi = 41.5 lbs/psi X 2103 = 50,67 rounded up 51 psi.

then there is an issue with low Aspect Ratio ( Hight/Width) tires as your 55 in the sises. They are given to high , a thing the tire makers dont want us users to know , but now you do. For 55% its not that bad but about 10% I would substract from the given maxload = 4 LI steps lower.

And at last , if you can give the GAWR's and GVWR ( Gross Axle/Vehicle Weight Rating) , those are the weights they use nowadays to fill in the formula to determine the advice pressure.

This would mean that your GAWR's are about 2 x 2203 lbs= 4406 lbs for both axles wich to my idea is pretty high.

And at last they calculated a pressure between 31 and 32 psi and rounded it to the highest psi so 32 psi.

But this last is yust playing in the margian.

hutch08 · February 3, 2015

Yep. http://toyotires.com/sites/default/files/page-files/TSD-12-011%20Replacing%20Tires%20on%20Light%20Trucks.pdf

Found this somewhere on this site, so putting it here as well. An LT will have run more pressure to obtain the same load rating as the P tire.

Thanks for the link, according to Toyo:

"Warning!

Please note that size-for-size; LT-metric tires require higher air pressure to carry equivalent loads of

P-metric tires, and that any failure to adjust air pressures to achieve the vehicle‟s load requirement will result in tire fatigue and eventual tire failure due to excessive heat build-up.

Due to the high PSI requirements of LT-metric tires, they may not be suitable for replacing O.E. P-metric tires because of the ride harshness that results from higher PSI requirements.

Load Comparison Example, P vs. LT:

P245/75R16 109 Max Load = 2,271 lbs @ 35 PSI

In order to carry the equivalent load, a LT245/75R16 LRC must be inflated to 50 PSI. Using this example, even LT245/75R16 Load Range D, or E must be inflated to 50 PSI to carry the P-metric load at

35 PSI. LT tires do not offer any benefits of being "heavy duty" when under-inflated.

Please note that size-for-size; LT-metric tires require higher air pressure to carry equivalent loads of

P-metric tires, and that any failure to adjust air pressures to achieve the vehicle‟s load requirement will result in tire fatigue and eventual tire failure due to excessive heat build-up. Due to the high PSI requirements of LT-metric tires, they may not be suitable for replacing O.E. P-metric tires because of the ride harshness that results from higher PSI requirements

Please note the considerably higher running temperature of the "under-inflated" (30 PSI) LT tire vs. 50PSI. Any increase in the tire’s running temperature will have an adverse effect on the long term durability and safe operation of the tire."

They even have a picture taken with infrared camera of a LT tire inflated to 30psi as well as 50psi showing the difference in sidewall temperatures.

hutch08 · February 3, 2015

And at last , if you can give the GAWR's and GVWR ( Gross Axle/Vehicle Weight Rating) , those are the weights they use nowadays to fill in the formula to determine the advice pressure.

GMC Sierra Crew Cab Short Box 4x4:

GVWR 3221kg / 7100lbs

Base Curb Weight 2394kg / 5278lbs

Max Payload 812kg / 1790lbs

Weight Distribution 58% Front / 42% Rear

jadatis · February 3, 2015

Googled your car and found this .

https://www.gm.ca/media/vehicles/current/gmc/savana/Savana_Brochure_Eng.pdf

There the 4x4 on page 18 ( Sierra| 18 in left bottom) saw GAWR 7100 with GAWR front 3950 lbs and rear 3950-4100 lbs.

These are to my opinion not the weights that will pusch on the axles when fully loaded at front.

Not even when only cabine is loaded.

Mostly front gets about half of GVWR so here 3550 lbs , or you must have a heavy motor on bumper.

Rear can have that 4100 lbs when loaded and towing , but at that moment the front axle is lifted up a bit so mayby even as low as 3000 lbs on front axle.

So I will fill in my motorhome calculator ( wich I use for other vehicles to) GAWR's of 3550 and 4100 for fully loaded, and have to make an estimation for normal use ( driver , co driver , and a little load ) by the empty weight and weight division you gave.

How did you come by those weight divisions, I see it often given , but wonder how it is determined . Is it the division of empty weight or Curb weight ( is there a difference?) . Explain the terms please.

Will calculate the weights for normal use here

Curb weigh 5278 lbs gives front 58% of that = 3061 lbs so rear 2217 lbs.

2 persons on front seat 400 lbs devided 2/3 on front and 1/3 on back axle= front 266 rear 134

On back seats of cabine it would devide 50/50% so you can add that if needed , Load in the box behind roughly only pusches on back axle so 0/100 division.

load on towbar depending on how much its behind the rear axle, say estimated division of -35%F/+135%R.

So weight in normal use ( correct me if I used wrong data) front 3061+266= 3327 lbs on axle.

Rear 2217+ 134+ 50lbs load in box= 2401 lbs.

In the spreadsheet I add front 10% and rear here 18% to give you some reserve for extra, so you dont have to fill up air when you do some shopping.

Will calculate and make picture and give it in next post .

But first will calculate your stock tires to see if the GAWR's filled in gives that 32 psi.

3950( gawr's) / 2 x 2400 ( maxload ) = about 27 psi so they must have been calculating different.

What they do in America is for P-tires that the maximum load is reduced by 9% so if you add 10% to that you get maximum load given.

This is the same as doing maxload /1.1) here 2400/1.1= 2182 then calculated the same way.

=32.34 psi so that must be about the calculation they used. Used the power in the official universal formula of Europe because America stepped over to it as late as 2006.

Lets calculate it with my on the road methode

3950-200( construction load Lc for 2 tires on the axle)= 3750

Maximum load 2 tires 2x2182=4364- Lcx2 of 200= 4164 lbs.

(3750/4164)x 35 = 31.52 so pretty close to the official calculated psi.

jadatis · February 3, 2015

GMC Sierra Crew Cab Short Box 4x4:

GVWR 3221kg / 7100lbs

Base Curb Weight 2394kg / 5278lbs

Max Payload 812kg / 1790lbs

Weight Distribution 58% Front / 42% Rear

So here the picture for loads I determined for your normal use. Used 10% reserve front and 18% rear , wich makes rear load% 85% so still no bumping and gives maximum reserve for incidental extra loading ( about 400lbs). Then advice as low as F 41 psi R 31 psi.

This is that low because of the lower weight then the car maker used ( gawr) to determine the advice pressure.

But also filled in some other axle weights

GAWR's filled in , these are the loads the tire makers use for their advice but now with 10% reserve added.

This gives F47psi R 52 psi

And then when not towing but fully loaded using GVWR of 7100/2 with 10% reserve all around 44 to my opinion realistic.

Now here the picture. but I placed picture ,pdf and spreadsheet under your username here on my one-drive that belongs to my hotmail-adress with same username as in this forum ( jadatis) , combine it yourselfes if you want to mail me, spamm machines cant this way.

To use the spreadsheet , first download it to your computer by right-clicking and choosing download, then after eventual virus-check open it in Excell or Open office CALC to use it. If you click left it is tried to open in the cloud wich programm cant handle it fully , because of some things I used in the spreadsheet.

https://onedrive.live.com/?cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC%211380

jadatis · February 3, 2015

Yep. http://toyotires.com/sites/default/files/page-files/TSD-12-011%20Replacing%20Tires%20on%20Light%20Trucks.pdf

Found this somewhere on this site, so putting it here as well. An LT will have run more pressure to obtain the same load rating as the P tire.

The temperature pictures of that article show that the most heat is procuced at the sides of sidewall , and that is what its all about.

those rings of the tire must stay below a sertain temperature , otherwise to much sulfur bridges are formed wich make the rubber hard an so at next bendings ( by deflections) they crack and damage the tire.

The tire at a sertain speed deflects and flexes back , so rubber bends to and fro, 10 to 20 times a second, wich produces heat.

The faster you drive the more heat is procuced.

The cooling down of those rings is done by the inside tire air and transported to the tinner parts of rubber and inside of rimm, At those places the heat energy is transported to the outside air.The faster you drive , the lesser deflection is allowed to keep the critical rings below that sertain temperature.

So my advices are for up to 160km/99m/h , wich is mostly enaugh, but if you drive faster, you need higher pressure to give tire lesser deflection so lesser heat-production.

Also notice at the pictures that at the middle where the letters TOYO stand the tire bends different so cooler temp on middle there.

So even this influences the temperature .

hutch08 · February 3, 2015

Thanks again for your help, I never realized there was so much calculations that could go into proper tire pressure haha.

If I recall correctly the factory TPMS is set to alarm at +/- 25% of the recommended tire pressure. So with a recommended 32psi, it'll alarm at <24psi and >40psi.

I can always have the dealership reprogram the set limits if I really wanted to.

I'm going to try bumping them up a bit and see how the truck rides. Once the snow melts I'd like to try the "chalk method" to see what kind of contact patch I actually have.

Was previously set to 32/32, now they're 41/38. It's currently -18*C (0*F) here so as the temperature warms the pressure will slowly increase as well so I didn't want to set them too high.

jadatis · February 4, 2015

I woulf sertainly have the TMPS reset , but only the recomended tire pressure to that 45 or what you think is best , having read all.

Then 25% of that is above and below the alarm is set of.

sometimes you can do it yourselfes , reading the manual.

This advice is for 18dgrC/65degrF given and so for colder temps like the -18C/0F you could calculate what the pressure would need with spreadsheet I made.

From the link to your personal map I made , you can navigate my complete public map , like in a forum.

If you go back to examples> motorhometire pressure , then you find the pressurecalculationwithtemp.

My idea is that colder ambiënt temp can have lower pressure, so more deflection, so more heatproduction, because the cooling down is better because of the larger temp differences between inside and outside tire.

If this is totally in line , I dont know ( yet) , can be that tire needs a bit higher pressure then the spreadsheet gives.

Did the work for you and made picture and placed the spreadsheet in the Hutch08 map from wich link already given.

If you want to try the chalktest then begin at to high pressure and go down in steps, untill chalkline wears evenly , and stop.

If you go on untill sides chalk wears of and centre stays, you are already to low.

A radial tire stays with its width on the ground in a large range.

So then line wears off evenly , but already to much deflection with to much heat production.