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Post Info TOPIC: how to calculate tire PSI to tire load


RV-Dreams Community Member

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how to calculate tire PSI to tire load


So when I converted my Ram 3500 diesel 4x4 DRW to SRW's, the tire pressure info on the door jam became null and void.  I was told an interesting formula for finding the proper PSI by a Nitto tire Rep.. Here it is......

Start with the tires sidewall information.  Maximum load rating and divide that by Maximum air pressure for the first part of the equation. For my tires it is 4,000 max. load divided by 80 max. PSI = 50 

2nd part of equation, the weight on each axle for the load weight to be carried by each tire.  My rear axle load weight is 6,400 lbs. or 3,200lbs.  on each tire.   front axle has 4,800 lbs or 2,400 each tire. 

Now for the rear tires PSI,   3,200 lbs. divided by 50 = 64 PSI rear tire pressure for 3,200 lb. load per tire    

For front tires PSI, 2,400 lbs. divided by 50 = 48 PSI front tire for 2,400 lb. load per tire

I have just under 30,000 miles on these tires. I have found this formula to be spot on. My tires look like they will last another 30,000 miles. My RV is a Tiger Bengal by http://www.tigervehicles.com



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Pretty darn simple weigh your rig and use the Load/Inflation chart specifically for your tire.

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2015 RAM/Cummins/Aisin/4.10's/3500Dually

2016 Mobile Suites 39TKSB3 "Highly Elited"

32,950# combined



RV-Dreams Community Member

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Dog ate it.  



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Sorry…that will not give you the right number. You need to get a copy of the inflation/load table for your tires. I took a casual glance at the inflation tables for our tires…and dividing the weight by the pressure gives different answers depending on which column from the inflation table you do the calculation for.



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That's funny I never said my tire size?  How and what did you look up?  LOL

FYI, My tire is a Nitto Ridge Grappler 305/65r 18 "F" rated. 3,970 Lbs @ 80 psi. spec's from...https://www.nittotire.com/light-truck-tires/ridge-grappler-light-truck-tire/

I looked but found no Nitto inflation/load guide but did find a Toyo, I'm told same maker as Nitto.

Looking up my tire size on page 25 in the Toyo site... https://www.toyotires.com/media/2125/application_of_load_inflation_tables_20170203.pdf

45 psi for 2680 lb. load

60 psi for 3280 lb. load.

They don't have an "F" rating for their 305/65r 18 their chart for a 'E" rated tire. Pretty close to the numbers I got doing the formula given by Nitto Rep.. I don't think Toyo's "E" tire has the same max. load or maxi. psi. If it did the PSI numbers would be the same.

 

 

 

 



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A quick tutorial on tire pressure.
www.discoveryparts.com/159__auto-racing-tire-pyrometers-

Please pay attention to the first line, it is spot on. For highway tires you need to take the temps after you have run a half hour or more and after you have run in a straight for a mile or more. After that, check your tire temps at the two edges and in the middle. If the middle is higher you're over-inflated, if the outsides are higher, you're under-inflated, if they're even across you're OK. If the two edges are not the same, or very close, you have an alignment issue you should address.

Here's a link to some tire pyrometers for your shopping pleasure www.longacreracing.com/products.aspx

They aren't quite as cheap as a chart from the manufacturer but it is more accurate. Personally I think the charts or the formula are probably close enough. Just my dos centavos and as always YMMV.

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Measuring tire temps may be something you do in a controlled situation such as a race, bottom line the only true way is to know your exact weight of the wheel positions. Then set your pressures according to the load and inflation chart. Keep in mind the PSI of the wheel position with the heaviest load becomes the setting for the other tires in an axle position. Therefore your front/steer axle may be one pressure, whereas the axle PSI could be different. And the trailer tires could be a different PSI.

Being in the tire industry for over 35 years, tire engineers may have checked individual tires for heat readings but at NO time did I ever have an engineer tell an end user (mainly trucking companies) they needed to set their pressures utilizing a heat reading or a formula other then the load and inflation chart.

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Your tires should be inflated to a load capacity that will carry the load of the certified GAWR for the axle, then add 10% for load capacity reserves. For the load capacity reserves you could use the loads the OE tires were providing as a guide.

I'm curious. Was the vehicle modification certified by a certified modifier? I bring this up because you have surely modified the vehicle by decreasing the ability of the vehicle's rear GAWR and its GVWR as certified by its manufacturer.  

You should read CFR 49 Part 567. Just put that in your search engine and it will take you to the regulation. It's in 7 parts. There are two versions. One by a law firm and the other by the government.



-- Edited by FastEagle on Friday 16th of November 2018 02:20:56 AM

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On an RV, axle weight sometimes varies considerably even on the same trip. Let's say you are leaving for a week of boondocking and have full fresh water and propane tanks (not counting vehicle fuel). In my motorhome all this weight is on my tag axle as it is at the rear of the vehicle. In my case, it's almost 1,000lbs! Let's say I enjoy a week or two off grid and use up most of my propane and fresh water, transfering all this weight to my waste tanks located in front of my rear axle for the return trip.

I think it's wise to adjust tire pressure to your actual maximum axle load in either configuration rather than pick say your starting weight, GVWR or UVW. My thoughts are that it's better to be a little over pressure than a little under pressure, but why adjust tire pressure to a maximum hypothetical weight you don't ever carry? If you ever do change your vehicle configuration and add weight, say you install a big solar system with heavy batteries, then you should reweigh. Are there any flaws in this logic?

Chip

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Like many things in life there are going to compromises in this area. No one is going to weigh their coach every time they leave on a trip. Now having said that 1,000 pounds on a 26,000 lb vehicle is only a 3.8% change in the weight. Any weight transfer between the fresh water tank and the black/gray water tank is going to affect the axle weights minimally as the change will cause the weight to rotate around the center of mass which will change slightly if the water as moved back a forth, but not materially. Now if your black/gray tanks are on opposite ends of the coach this change could be material, but I don't think that's the case.

As to inflating the tires to the GAWR for the axle that's going to have you over-inflated in every case unless you are at the GAWR. This causes two issues. First, an over-inflated tire has less grip as it is riding on the center of the tire. The other issue it causes is poor tire wear. Since the tire is riding in the center, the center of the tire will wear faster, and you'll have no tread left in the center and too much tread left on the outside. While I agree that over-inflation is preferable to under-inflation because under-inflation can cause blowouts why not try to get it as close as possible.

As to using tables as opposed to temperature I said that was probably the easier and most practical way to go, but how do you think they come up with those tables? Tire temps. Not my opinion, it's the unanimous opinion of Steve Smith who wrote a book on race cars suspensions, Herb Adams who was a suspension engineer for GM, and Jeff Speer who is a tire engineer for Hoosier tire. This methodology applies to tires in any situation, racing, street use whatever, the tires always perform the same basic function just sometimes it's just more extreme.

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I personally add 5psi to the chart for rears and 10psi to the fronts. Doing so I would get 120k out of a set of Michelins on my 98 2500 4x4 RAM. 100+ on BFG's.

Per GY when going up in load range use the chart and add 5psi on your trailer tires.

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2016 Mobile Suites 39TKSB3 "Highly Elited"

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RV-Dreams Community Member

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The formula has my tires tread still looking new after 30,000+ miles. That's all the verification I need.

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This is why in the late 90’s most major tire manufacturers changed from suggesting to set the tire pressure for the appropriate weight to now inflating the trailer tires to the max. Too many uninformed or tribal knowledge people muddied the water. Over inflating the tire does NOT increase the carrying capacity of the tire. If the tire calls for 80 PSI to achieve it’s stated maximum carrying capacity adding 5 or 10 psi does have no effect in enabling the tire to haul more. All it does is invite irregular wear.

Also people as va general rule do NOT regularly check their air pressure so that is another reason to air the tire to its maximum, something I personally don’t recommend but if you don’t check the air pressure then go for it.

Having worked for the three largest manufacturers at one point in my career there are exceptions, but it is for a very small window of applications.

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Is this thread about a common dually pick-up truck? If so, its tire inflation pressures are product of collaboration between the tire manufacturer and vehicle manufacturer. The ruling body – NHTSA/FMVSS - has given the vehicle manufacturer sole responsibility for setting the vehicle’s recommended cold tire inflation pressures for the original equipment tires. In fact, in the FMVSS they have directed them to do so. Deviations may be found in the vehicle owner’s manual. Options are available from the recommended to the sidewall max.

The standards for replacement tires are derived from the OE tire parameters. In short, they must be able to provide a load capacity, via inflation, equal to or greater than the OE tires provided. 

The following quote is from the USTMA. They set our tire standards.

“Inflation pressure recommendations may also be determined based on the tire manufacturer’s specifications, which define the amount of inflation pressure necessary to carry a given load. These inflation pressures may differ from those found on the vehicle tire placard or certification label."  

"However, never use inflation pressures lower than specified by the vehicle tire placard, certification label or owner’s manual. Nor should inflation pressure exceed the maximum pressure molded on the tire sidewall.”

 



-- Edited by FastEagle on Saturday 17th of November 2018 12:07:35 AM



-- Edited by FastEagle on Saturday 17th of November 2018 12:10:15 AM

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FastEagle, thanks for asking. The answer your question, No. This thread is not about a common dually pick-up truck.
FYI, I was given a simple formula for air pressure per tire load by a tire Rep. It worked well for me and thought I'd pass along the info. I never saw the formula posted before and thought it an alternative to other methods. I used my truck tires as an example, because I changed tire size an had no reference point.
Furthermore, why folks are hacking my thread about the way they find tire pressures is beyond me ? This tread has now been hacked to the point where the formula is totally obscured. Your confusion is a perfect example.



-- Edited by sourdough on Saturday 17th of November 2018 12:39:02 PM

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sourdough wrote:

FastEagle, thanks for asking. The answer your question, No. This thread is not about a common dually pick-up truck.
FYI, I was given a simple formula for air pressure per tire load by a tire Rep. It worked well for me and thought I'd pass along the info. I never saw the formula posted before and thought it an alternative to other methods. I used my truck tires as an example, because I changed tire size an had no reference point.
Furthermore, why folks are hacking my thread about the way they find tire pressures is beyond me ? This tread has now been hacked to the point where the formula is totally obscured. Your confusion is a perfect example.



-- Edited by sourdough on Saturday 17th of November 2018 12:39:02 PM


This statement is what caused the tire pressure to be so puzzling. (So when I converted my Ram 3500 diesel 4x4 DRW to SRW's, the tire pressure info on the door jam became null and void.)

How was you able to maintain the vehicle certified GAWR for the rear axle? If you cannot provide a rear axle RGAWR, equal to the vehicle manufacturer's certified RGAWR, the vehicle no longer meets NHTSA safety standards as described on the vehicle's federal certification label. The axle information on the original federal certification label can only be changed by the vehicle manufacturer or a certified vehicle alterer/modifier. 

If you have modified the original rear axle from dual wheel to single wheel the axle is probably still capable of carrying its certified load. IF, you can find single tire/wheel assemblies that will have the capability to support the vehicle certified axles. The rear axle braking difference between a dual & single wheel configuration is another safety consideration in the new weight calculations.

Without a known axle rating, recommended cold tire inflation pressures cannot be properly set. 

 

 

 



-- Edited by FastEagle on Sunday 18th of November 2018 04:52:57 AM

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RV-Dreams Community Member

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Note to self.... Refrain from further posting.
BEAM ME UP SCOTTY

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In reviewing FastEagle’s posts, why wouldn’t you want your tires inflated to their maximum load carrying capacity?

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^^^^^ Harsh ride, poor stopping, center tread wear to name a few! Blindly run MAX if you want but if you want the BEST tire life, ride and stopping ability use the load/inflation charts. Most OE tires need to be ran at MAX simply because that is how they rate their RV's.

Mobile Suites run GY "H" tires on all their newer RV's. Some need 125psi but MOST don't, especially triple axle units.

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2015 RAM/Cummins/Aisin/4.10's/3500Dually

2016 Mobile Suites 39TKSB3 "Highly Elited"

32,950# combined

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