tire pressure

[TerryandKim]

I bought new tires for the truck switched from the P rated that was on the truck when I bought it to LT tires. New tires are rated for 80lbs pressure. Question: Do I run the tires at 80lbs all the time or just when towing the travel trailer? What would a normal pressure be for driving with empty truck? probably a silly simple question for some of you, but I don't know ... Thanks!

Michelin LTX MS2 245/75/16

[TerryandKim]

Thanks for all the advice! I'll go with what the tire manufacturer gives as a minimum pressure for tire according to weight load. This question got quite the conversation going tho!

[DiskDoctr]

jadatis wrote:
@Terry And Kim

No you want to devide the whole weight of car over 2 tires but a car has 4 tires so devide that 5740 lbs by 4 so 1435 a tire.
This would give 40 psi in my list.
But the weight is unequally devided over front and back , so that is why my first given advice is different front and back.

I also daubt that if you are fully loaded , that your weight is that 5740lbs. Practice prooves that weights are always yudged to low.
Best advice would be, and probably also already given in this topic, to weigh per wheel in the loading you drive, and use those weights to determine the needed pressure. For each axle use the highest weight for calculating


@ Diskdoktr

Up to 2000 the tire and car makers also gave these low advices, so they thought it to be save too.
And also Goodyear still gives a list for ST tires that goes as low as 15 psi still on a page with Pressure loadcapacity lists.
Then I am refering to this page
The ST list at the top
I already mailed them about it to be verry unsave because they calculate these radial tires with the power 0,585 wich is meant for diagonal tires. But they are hiding behind TRA and sticking their head in the sand.


I think the Ford/Firestone affaire, in wich more then 100 people died because of roll over accidents courced mostly by blewing rear tire had a large impact on how car and tire-makers handled tire pressure advice.

What I concluded of that affaire, is that the tires maximum load was given to high for the offroad looking Firestone tire , because the profile blocks that cover a part of the sidewall made the sidewall less flexing allowed then a normal car tire of same sises and AT-pressure. So not the wrong calculation in that time for P tires with power 0,5 in the universal formula, wich was patced by adding 10 % to the loads for SUV, and not going lower then 26 psi.
European ETRTO stops at 21 psi, but if you would have the to nature ideal formula, wich my calculation comes close to, you could even go as low as zero pressure if the calculation gives it.
In Carting and for verry light vehicles with oversised tires they even go as low as 18 psi or even 12 psi with no blowing tires.
Exampels I came by in time , Spider ( 3 wheels)& Westfield.

To my opinion in some cases they over-reacted.
As late as 2005 American TRA Stepped over to the 0.8 power that European ETRTO used since about 1970 for all kind of tires, but only for P-tires and XL//. The Ford/Firestone affaire played about 1990 to 2000. The closer the power to 1 the lower loadcapacity for the pressure, or the other way around the higher pressure for the same load.

So I am not planning to burn my spreadsheets , have to burn the computer for it.
Always advice to use a reserve for things like , pressure loss in time, misyudging or misreading of weight , misreading of pressure , incidental extra load or loadshifting , unequally devided R/L, etc.

Actually, the Firestone incident involved underinflated tires, per VEHICLE MFR stickers, in contradiction to Firestone's recommendations on the tires.

It caused excessive flex of the tires that lead to a high number of rollovers.

Unless you are thinking about the blowouts on Jeeps from the Fiberglass cords in some offroad style tires.

Under inflation (gross) leads to handling issues, not just blowouts.

Also remember that weight and stresses can change drastically while in motion- especially with a dynamic load like a trailer/rv, and even more as you approach limits of hauling.

Curves, bumps, hills, uneven/unlevel roads, braking, load balance and shifting, and many other factors need to be considered.

The kind of homegrown spreadsheets as you tout can cause great danger. Besides, unless you are trying to "prove" you can survive with lower than prudent pressures, what do you expect to gain? Mpg? Bouncy ride? Bulging sidewalls? Uneven tread wear? Erratic handling?

Look, you sound like a smart fellow...be wise,too. Don't encourage people to make a habit of running tires at marginal pressures, someone might get hurt.

[jadatis]

@Terry And Kim

No you want to devide the whole weight of car over 2 tires but a car has 4 tires so devide that 5740 lbs by 4 so 1435 a tire.
This would give 40 psi in my list.
But the weight is unequally devided over front and back , so that is why my first given advice is different front and back.

I also daubt that if you are fully loaded , that your weight is that 5740lbs. Practice prooves that weights are always yudged to low.
Best advice would be, and probably also already given in this topic, to weigh per wheel in the loading you drive, and use those weights to determine the needed pressure. For each axle use the highest weight for calculating


@ Diskdoktr

Up to 2000 the tire and car makers also gave these low advices, so they thought it to be save too.
And also Goodyear still gives a list for ST tires that goes as low as 15 psi still on a page with Pressure loadcapacity lists.
Then I am refering to this page
The ST list at the top
I already mailed them about it to be verry unsave because they calculate these radial tires with the power 0,585 wich is meant for diagonal tires. But they are hiding behind TRA and sticking their head in the sand.


I think the Ford/Firestone affaire, in wich more then 100 people died because of roll over accidents courced mostly by blewing rear tire had a large impact on how car and tire-makers handled tire pressure advice.

What I concluded of that affaire, is that the tires maximum load was given to high for the offroad looking Firestone tire , because the profile blocks that cover a part of the sidewall made the sidewall less flexing allowed then a normal car tire of same sises and AT-pressure. So not the wrong calculation in that time for P tires with power 0,5 in the universal formula, wich was patced by adding 10 % to the loads for SUV, and not going lower then 26 psi.
European ETRTO stops at 21 psi, but if you would have the to nature ideal formula, wich my calculation comes close to, you could even go as low as zero pressure if the calculation gives it.
In Carting and for verry light vehicles with oversised tires they even go as low as 18 psi or even 12 psi with no blowing tires.
Exampels I came by in time , Spider ( 3 wheels)& Westfield.

To my opinion in some cases they over-reacted.
As late as 2005 American TRA Stepped over to the 0.8 power that European ETRTO used since about 1970 for all kind of tires, but only for P-tires and XL//. The Ford/Firestone affaire played about 1990 to 2000. The closer the power to 1 the lower loadcapacity for the pressure, or the other way around the higher pressure for the same load.

So I am not planning to burn my spreadsheets , have to burn the computer for it.
Always advice to use a reserve for things like , pressure loss in time, misyudging or misreading of weight , misreading of pressure , incidental extra load or loadshifting , unequally devided R/L, etc.

[TerryandKim]

Made you a list with yet another spreadsheet with my calculation so to my opinion the right load capacity's for the pressure .
Made it per tire and only LBS but one in KG and one in LBS.

"per tire" ... so my truck weighs approx. 5740lbs when loaded with family/stuff and trailer. Is "per tire" meaning I would use the chart at 2870 (5740 divided by 2 tires = 2870 per tire)? According to the chart at 2865 I would use 75lbs air?? Your chart is actually asking for 5lbs more than what Michelin gave me. Thanks!

[DiskDoctr]

jadatis wrote:
The E-load tires you now have only need little more pressure then the P-tires wich are B-load tires with pressure needed for the maximum load of 35 psi.

But when you search for the pressure advices, on same plate is mostly the Gross axle weight ratings ( GAWR) and GVWR ( V for Vehicle).
Then read from tires the maximum load ( googled and found 109 Loadindex wich is 1030kg/<>2260lbs a tire for P tire in 245/75R16) and check if it gives AT 80 psi on sidewall.
Then to make it complete give speedcode ( probably Q for up to 99m/h)

Then I will calculate the needed pressure for you with use of one of my made spreadsheets , In wich I use an even saver formula then the tire-makers use. Also with some reserve for towing.

I expect the outcome to be as low even as about 40 psi rear when towing. Front then can be even lower because GAWR front is mostly lower then rear and when towing front axle is lifted up a bit by the weight on the pin.

Do NOT tow with 40psi in 80psi fated tires!

That is bad and dangerous advice, prone to get someone hurt. Time to burn your magical fairy-powered spreadsheet :R

You're playing with people's lives, please be responsible.

[voodoo101]

darsben wrote:
When you have weighed the truck you can then find the TIRE MANUFACTURERS chart that will give you the correct pressure for the tire based upon the weight.

Agree and have passed similar advice many times. However I prefer to refer to the pressure based on weight as the "minimum safe pressure" for the load rather than the "correct pressure". A nuance but emphasizes the need to keep at least that pressure vs making it the target pressure. A point you noted in your following sentence. Yes, I picked a nit. Bill

[jadatis]

Read the maximum load wrong and used 1700 kg so gave to low answers.
So correction Front 48 psi / 3.4 bar and rear 57 psi/3.9 bar
sorry for the mistake but rest of my answer still stands.

Then about the list you got from Michelin.
Calculated it back to be using the power in the universal formula of 0.7 wich gives to much deflection in the lower load /pressure range.
If you whould have asked Michelin Europe they would have given you a list made with power 0.8 wich is save enaugh for P-tires and XL but even also gives for LT tires in the lower load/pressure range more deflection.

Made you a list with yet another spreadsheet with my calculation so to my opinion the right loadcapacity's for the pressure .
Made it per tire and only PSI but one in KG and one in LBS.

[jadatis]

already replied to your personal message, but here the copy of that.

Filled your data in my motorhomeRV tirepressure calculator.
Gave Front 39 psi/2.7 bar and rear 46 psi/3.2 bar.
Edit: read wrong maximum load of tire , corrected in my next reaction. Front 48 psi rear 57 psi .
Used front 5% reserve to GAWR because front seldomly overloaded.
And rear even 18 % reserve to GAWR because rear often overloaded.
This makes if extact GAWR and equally devided R/L the load to be 85% of the load the pressure is calculated for , wich then would give still acceptable comfort and gripp.

You can drive up to that 170 km/106M?/h( speedrated R) fully loaded with trailer behind so higher is not needed.
And calculated with my extra save formula that takes care of the same deflection of tire over the whole range of pressure/loadcapacity.
If you drive empty you can use 39 all around or mayby even lower.


The GAWR's together 7350 wich is 1150 lbs more then GVWR of 6900 lbs.
This large difference is probably tipically American to cover the 10 to 15% on towbar , In Europe its mostly only about 500lbs because here towbarweight must be between 1% and 10% and is mostly a poor 5%.
Or mayby its possible to upgrade GVWR to 3100kg/6900 lbs.

For the trailer in your next reaction I have to know the tire data.
Mind ST is for only up to 65m/h but lately made complicated list.
See next topic for that on RV forum.
comparing tires for replacement on Trailers with in last reaction about the spreadsheet to make list...

Greatings from a Dutch pigheaded self declared tire-pressure specialist
Peter

[TerryandKim]

hello jadatis, Just seeing your message about your spreadsheet.
I found this on the door plate and tire.

GAWR Front:1633kg/3600lbs Rear: 1701kg/3750lbs
GVWR 2812kg / 6200lbs

Tire max load: 1380kg / 3042lbs speed rating R

thanks, Terry

[TerryandKim]

I phoned Michelin directly. The very helpful customer service person gave me a list of what pressures to use with particular vehicle weights for my tires. example: 50psi at 2205lbs, 55psi at 2335lbs, 65psi at 2623lbs etc up to 80lbs. sounds like I'll never need the 80lbs though. I'll go with those recommendations from Michelin. Now I just have to make sure the wheels will take the pressure, if not, I'll get some replacement rims.
Thanks for the help guys!! What I didn't know before, now I do!

[jadatis]

The E-load tires you now have only need little more pressure then the P-tires wich are B-load tires with pressure needed for the maximum load of 35 psi.

But when you search for the pressure advices, on same plate is mostly the Gross axle weight ratings ( GAWR) and GVWR ( V for Vehicle).
Then read from tires the maximum load ( googled and found 109 Loadindex wich is 1030kg/<>2260lbs a tire for P tire in 245/75R16) and check if it gives AT 80 psi on sidewall.
Then to make it complete give speedcode ( probably Q for up to 99m/h)

Then I will calculate the needed pressure for you with use of one of my made spreadsheets , In wich I use an even saver formula then the tire-makers use. Also with some reserve for towing.

I expect the outcome to be as low even as about 40 psi rear when towing. Front then can be even lower because GAWR front is mostly lower then rear and when towing front axle is lifted up a bit by the weight on the pin.

[Old-Biscuit]

Good choice in gong from 'P' rated tires to 'LT' due to towing BUT
I see new rims in your future :B

I run 80# rear when towing because I am at MAX Load Rating on rear tires and need the max air pressure.
I air down to 55# rear cause that is what I need in tires when not towing

Fronts stay at 55# cause weight doesn't change towing or not

BUT those air pressures are what give me good wear, traction & stopping power on my truck and set up. YMMV

[Alan_Hepburn]

If your truck came from the factory with P-rated tires there's a good chance the wheels were matched to the tires. They probably can't handle the higher pressure that you can put into the LT tires - you need to dismount a tire and look for the pressure rating of the wheel - usually stamped into the wheel on the inside.

[DiskDoctr]

john&bet wrote:
I lower the air pressure in my "E" tires on my 2500 to about 65 rear and 60 front when not towing. I up my front to 70 and rear to 80 when towing.

Pretty much the idea. When upgrading tires a fair amount, you want to be careful not to overpower your suspension when not hauling a load.

Lowering the pressure unloaded a bit allows harmony between tires, suspension, and road, yet still enhance performance when loaded.