Tire Tech: Understanding the relationship between PSI & Load. LT265 and LT255 data!

As I've been looking into replacing my OEM BFG Rugged Trail P265/70R16 tires with something a little larger and more aggressive. I've been looking at Duratracs in LT265/75R16 and LT255/85R16 as options. I'm leaning more towards the LT265/75R16 option.

One thing I've been trying to wrap my head around is the choice between the Load Range C and Load Range E options on that Duratrac LT265/75R16 tire. Many will say the LR-E tire is "too much tire" for our Tacomas, with its 3,415 lb. Max load @ 80 PSI rating. I'm not even sure my TX Pro wheels are rated for 80 PSI (need to research that.)

The Load Range C version maxes out at 2,470 lbs. @ 50 PSI. Given the GAWR for the rear axle is 3,110 lbs, the LR-C tires are "overkill" with their combined 4,940 lbs. of load capacity. That is plenty for our Tacomas, but maybe the LR-E tires are going to be "tougher" for off-road situations. Naturally, I expect the LR-E tires to ride rougher than their LR-C counterparts.


One thing that has been much harder to find though is what air pressure I'd need to run for my truck. Searching the web comes up with all kinds of methods, from the "chalk test" to various formulas. One thing I have learned is that at any given pressure, an LT265/75R16 tire in either LR-C or LR-E has the SAME EXACT LOAD CAPACITY! Yes, you read that correctly. An LR-C tire at 50 PSI will carrying 2,470 lbs, and an LR-E tire at that pressure carries the same 2,470 lbs. Where the LR-E gets its extra capacity is from the higher pressure it can handle, up to 80 lbs.

So, finding the "max load" of a tire is pretty easy. Just go to Tire Rack's website and you can find it. But Max Load is pretty useless for everyday driving. In searching the web, I found an excellent resource on Toyo's website. It is a PDF that can be downloaded from here: http://toyotires.com/tires-101/load-and-inflation-tables

On page 14 of the PDF, there is a table from the TRA (Tire and Rim Association.) This table, for both P-metric and LT-Metric tires, shows the load carrying capacity of the various sizes of tires available. These are standards, not brand specific, thus an LT265/75R16 LR-C tire made to the specification from any manufacturer will have these characteristics.

I have also learned that for any given pressure, a P-Metric tire will carry more weight than the same size LT-Metric tire. This will become important later on.

I found the LT265/75R16 data, and plotted it out and did some extrapolations. The load carrying capacity of the tire is nearly linear:



As the data from the Toyo/TRA PDF file only listed the load carrying capacity in 5 PSI increments, I extrapolated the data to find the load carrying capacity at single PSI increments. In creating the table, I took the difference in weight capacity from each published figure, and divided it by 5. While the graph is nearly linear, it isn't totally linear. So I extrapolated the data based on the change for each 5 PSI range. For example, the change in weight carry capacity from 40 to 45 was 36 lbs. per PSI, but the difference from 65 to 70 is 21 lbs. per PSI.



All of the information is "use at your own risk". Also, the section in red was extrapolated using the highest drop in weight capacity (38 lbs. per PSI), as the lowest published figure for LT tires was 35 PSI. I accept no responsibility if that information proves to be inaccurate!!!!


Now that we have this chart to use, we can figure out about what PSI one would need to run on an LT265/75R16 and be near the factory specification. The sticker in my 2011 Double Cab 4x4 recommends the front tire should be at 29 PSI and the rear should be at 32 PSI with a P265/70R16 tire. As that same Toyo/TRA PDF has the specs for this tire size, I compared it with the LT265/75R16 specs:


As you can see, the P-Metric front tire at 29 PSI will carry 2,216 lbs and the rear tire at 32 PSI will carry 2,326 lbs. To get the nearly equivalent weight capacity from the LT265/75R16 tire, I will be between 43 and 44 PSI in the front and 46 and 47 PSI in the the rear. If your tire shop installed these tires for you and ran "factory pressures" or just set it at 35 PSI, you would be well below the necessary PSI for the new tire! That is increased wear, as well as shortened life due to heat cycling, etc. Also potentially deadly at highway speeds with a tire failure (think Firestore and Ford Explorer....)

As I know many also like to run the LT255/85R16 tire size, I've included the same graph and table below for reference:

LT255/85R16 Graph:


LT255/85R16 Table:



If you happen you know your actual front and rear axle weights (from a truck scale, etc.) you could really fine tune the PSI for your tires.

 

Wow thanks for the info. I used the info and a simple calculation and came up with my max tire pressure of 33 in the front and 37 in the rear rounded up of course!

 

There's more to E tires than just load rating. I got mine to avoid rock cuts; some of the roads around here can have some sharp rocks turned up right after the grader goes over them.

 

True. In order to get to D or E load ratings, the tire has to be built tougher. That means (usually) more plies in the tread and sidewall area. One would hope that more plies means better cut resistance, etc. But not always, it may be that the ply count is the same, but the makeup of those plies is different.

But more plies also means a less flexible carcass, less deformation around obstacles at a given pressure, etc. Thus why some people note a much harsher ride on an E load tire vs. a C.

But I think most people get confused about load ratings, I know I was before researching this. I didn't know that the load rating at a given PSI was identical for a C, D or E load tire. The extended load capability comes from the tires ability to withstand higher pressure (and the wheel's ability to handle the pressure too*).


*Trailer wheels have max PSI ratings, so I'm assuming that passenger/light truck wheels have limits too. For example, my 5 spoke aluminum trailer wheels are rated at a max of 80 PSI, but some trailer wheels will go up to 110 PSI.

 

One hell of a scientific breakdown. I applaud your research. One vote for sticky!

 

you MUST be an engineer

rep to ya

 

Ha, you'd think! I did graduate a long time ago with a BS in Physics, but my current job is "insight analysis" for a global beer brewer of the "green bottle" variety. So I do work with numbers, a lot, every day.....

 

What a post!

 

Thanks for the great write up! I looked high and low for a complete chart and it seems the only place I could find it was Toyo Tires. Congrats to them for posting it up (even if it is a bit hard to find). One thing to note is that the ratings for the stock P265/70/16 tires is actually a bit lower than you posted (according to Toyo's site). It seems that when P rated tires are used in SUV and Light Truck applications they are "de-rated" by dividing the load capacity by 1.1 at any given PSI. So the actual factory load capacity would be a bit less than stated above. Also, not sure what Toyota is doing to figure their tire pressure (other than maybe more air to gain more mpg). I have an 02 Tundra TRD OR AC which is pretty much the same size as the current Taco (about 3" wider and 450# more gvw). It came with 265/70/16's with a door sticker of 26psi front & 29psi rear for a load of 1913F and 2014R per tire or total combined of 7855# on a GVW of 6040#. I have yet to find any charts that show loads for LT tires at below 35psi so can only assume the mfgs don't want to show a lower psi due to heat issue etc. Not sure on that one. The 265/70/16 D rated AT's on my new pro came with a rec setting of 42psi which would allow for a gvw of just over 8000# so close to the same load rating as speced on my Tundra.
LT265/70/16's at 35psi are rated at 1820# so should work fine for an empty truck as they come out to almost the same +30% of max gvw as the original tires. Just a thought. For those who want to look at it all, here is the link to the entire PDF from Toyo which shows most all normal types and sizes of tires.


Note: The link says you need a front end ap to run the program...just click cancel and it works fine.

http://toyotires.com/sites/default/files/page-files/LoadInflation_Table_P-LT_102913.pdf