How much PSI?

[Colt45ws]

Originally Posted By: CapriRacer
According to Tire Guides, a 1991 K1500 came with either LT225/75R16's inflated to 50 psi, or LT245/75R16's inflated to either 50 psi or 45 psi depending on the GVW.

Please note that there was no listing for an LT225/75R16 at 45 psi. So I am going to do the calculation both ways.

The load carrying capacity of an LT225/75R16 at 45 / 50 psi is 1790# / 1940#.

In order to carry the same load, an LT245/75R16 needs to be inflated to 38 / 43 psi.

- HOWEVER -

There is a school of thought that if you have enough load carrying capacity, it is better to match the spring rates of the tire by using the same pressure as on the placard. So I recommend using whatever is on the placard - and please check: No guessing.

Then comes Capri with actual expert info. Thank you. An asset to the site.

 

[TiredTrucker]

Get a load / Pressure chart from the maker of the tires. I do that all the time. I run LT 275/70R18's on my 3/4 ton and there is no way I am going to use the OEM recommended pressure when empty or light loaded. I suppose I could if I wanted to beat my suspension to death. I check my axle weight at a local scale and I follow the load / pressure charts from the tire maker and I manage to get very good performance and longevity from my tires. After all, it was the tire maker that did the R&D and knows what the tire should be running, not the car or pickup builder. And I have yet to find an instance in 45 years that the load on the tires requires the max inflation on the sidewall of the tire. I suppose if overloaded, but then, stupid is as stupid does to quote the great philosopher Forrest Gump.

 

[CapriRacer]

Originally Posted By: TiredTrucker
Get a load / Pressure chart from the maker of the tires. I do that all the time. I run LT 275/70R18's on my 3/4 ton and there is no way I am going to use the OEM recommended pressure when empty or light loaded. I suppose I could if I wanted to beat my suspension to death. I check my axle weight at a local scale and I follow the load / pressure charts from the tire maker and I manage to get very good performance and longevity from my tires. After all, it was the tire maker that did the R&D and knows what the tire should be running, not the car or pickup builder. And I have yet to find an instance in 45 years that the load on the tires requires the max inflation on the sidewall of the tire. I suppose if overloaded, but then, stupid is as stupid does to quote the great philosopher Forrest Gump.


This post has some minor issues - and I hope TT realizes I am not trying to pick on him. I'm only interested in getting all the details correct.

First, the charts issued by the tire manufacturer are actually the charts issued by tire standardizing organizations.

Barry's Tire Tech - Tire Standardizing Organizations

Second, those charts are MAXIMUMS, not recommendations. Vehicle manufacturers take those charts and use them to size the tire - and typically, they chose a tire size and inflation pressure that is greater than the maximum load for the truck - and that's what they put on the vehicle tire placard.

So if you are changing tire size, you need to calculate what the vehicle manufacturer used as a max tire load. Even if you know the corner weights, you need to know how much over capacity the vehicle manufacturer factored in their pressure specification and take that into account. And I hope everyone understands that vehicle manufacturers have put a lot of testing into how they chose their tire size / pressure procedure and they verify each and every one by actually testing the vehicle in question at the size/pressure specified.

Further, it's been found that vehicles prefer the tire to have the same spring rate as the original - and for practical purposes, a tire's spring rate is a function of it's inflation pressure. This is also something vehicle manufacturers have found in their testing.

However, there is a problem with large capacity vehicles such as pickup trucks and full sized vans. Empty vehicles don't load the tires the same (not to mention the front to rear weight distribution changes). Some vehicle manufacturers will specify an empty tire pressure in addition to the fully loaded one - some don't. But all of them test their vehicles at anything they specify. So be very careful if want to deviate from what the vehicle manufacturer specifies.

 

[zeng]

Recommended tire pressure for a vehicle is typically always given by vehicle OEMs, never by tire manufacturers who doesn't know load/weight applicable for front and rear axles of a vehicle.

This OEM recommended tire pressure psi reading always has a specific psi reading (to nearest 1 psi) as often found around the door/placard .........
and never given in a range of psi.

Despite differing loads/weights between front axles and rear axles, majority of OEMs specify one specific psi reading for both axles (probably true in OP's case) , though minority of OEMs specify one specific psi reading for front axle and another specific psi reading for rear axle.

JMHO.

Ooops .......

 

[slo town]

Originally Posted By: BigCahuna
Sorry MR expert. Maybe they can change the name of this sight to yours?,,


That fact is, BigCahuna, your statement was wrong. It has nothing to do with being "expert".

Scott