Wear and inflation rate is going to depend on the load placed on the tire. What would wear fine on my JK at 30psi will not wear the same on my Dodge Ram at the same inflation rate. However, running the same tire at 45psi on my truck wears evenly just as it does on my JK.

 

Thank you very much for the detailed information.

You have cleared this up well.

I'm figuring after all of this the D rated tire would make sense for me. I put most of my miles on the highway and being able to run it at a bit higher pressure than an E rated tire would benefit me more. I'm sure other's would come to a different conclusion based on their needs and driving habits.

I think from what I'm hearing that for the most part the E rated tire for the same model tire could produce a very similar ride to the D rated tire but at a lower pressure. I would presume the wear would be ok but not as good as the D and the mpg would be worst due to running at a lower pressure.

Feel free to correct any of my assumptions!!!!

 

Despite putting different loads or using different inflation rates, tires are designed to have a specific size contact patch. For instance, a tire with a 60 square inch contact patch would support 1800lbs inflated to 30psi, 2400lbs inflated to 40psi and 3600lbs inflated to 60psi. It's not really over or under inflated at those rates, just inflated to a point that it handles the load and maintains the appropriate contact patch. Expanding the contact patch or flotation is what will reduce efficiency.

 

LOL

The problem is that its just not simple.



We WANT it to be simple and easy to remember, like get Load D, etc.

The reality of it is that there are so many variables to what goes into a tire's function, that there is no universal rule you can follow.

What I so far have found is that the 35% rule, applied to a tire you are considering, is typically ball park. (See prior post in this thread)

After that, some tires run fine aired down or inflated more than required, etc, and others have a narrow window of inflation for a particular load to work well.

After you HAVE the tires, a chalk test is probably one of the most useful things you can do.

Essentially, you chalk the tread, and drive over a contrasting colored surface (White chalk/drive on blacktop, etc)

This tells you where the chalk is worn off, and deposited.

Generally, if the chalk is evenly distributed, the tire's contact patch is appropriate, and the PSI is working for you.

If the center of the tread is not depositing/wearing as much, the tire is under inflated.

If the chalk is wearing/deposited from the center more than the sides of the tread, the tire is over inflated.

There may be a RANGE of psi that can retain an even pattern though, and for higher speeds and more precise handling, going to the upper end of that range will provide improvements, and the lower end of that range will typically soften the ride.

The above is a way to optimize what you have.

The fact remains though that the RESULTS will NOT be the same for all tires...the individual size, load range, rig its on, type of construction, etc...will change what works for YOU.

 

Hey Teej,

I agree with what you wrote and the chalk test is ultimately the best tool to dial a tire in as to inflation rate. Where I don't exactly follow along is that we have the ETRTO and Service description numbers that get us infinitely closer to making an informed decision on tires, and very few in the US, outside the manufacturers,uses them. I certainly don't believe it to be an end all lock, but it does provide a pretty good window of where things fit, and well beyond that of C/D/E load ratings.

 

Teej and JPop: Thanks for a lot of good information and discussion! I, too, am struggling with the load-rating issue in regard to the replacement tires I'm considering.

Steve

 

I'm still not completely clear on this subject!!

I have done some calculations with this information and based on the specs from a couple of tires I have looked at the pressure in an E rated tire will be higher than the pressure in a D rated tire!! This assumes the pressures from previous posts as the tire pressure specs aren't listed by the manufacturers. If the pressure is higher and the tire material is stiffer (I assume an E is stiffer than a D) then the tire would have to have a rougher ride!!! If it worked out to be a lower pressure then I would believe an E could ride like a D. This is shown to have a lower pressure above based on the ISO tables.

I think the best information that I have some trust in is the ISO load rating table.

Now the problem is to find the load ratings for the tires I'm interested in!!

 

I bet a case of beer people would fail a blind test of D vs E for the same tire at same PSI. It's like the butt dyno ... useless.

 

If I was thirsty and wanted to steal your beer I'd take that bet in a heartbeat.

The effective difference between some D and E rated tires at the same inflation rate could be as dramatic as doubling your spring rate from 500lbs per in to 1,000lbs per inch.

Each tire inflated to a point where they are having the same load index, now that would be a completely different story.

 

Yeah BC, That is an awesome reference document to have around and there are some other ETRTO that basically have the same data.

In a nutshell, I think we do the whole community a service by not mentioning outmoded C/D/E ratings and just speak of ETRTO load index numbers which tell a much more complete story. There available from nearly every tire manufacturer and with the inflation rate data will get you pretty close to appropriate inflation rates.