Mapper,
Not exactly the size you're after, but I got Cooper AT/3s in 265/75R15 (=30.6x10.4).
Yes, I noticed that. For that size I'd probably prefer the 31x10.5 for a load range C LT tire. Lots of options available at that size. I keep waffling on this. I'm hoping to get to the JY this weekend and see if I can scavenge up some decent alloys.
I want to thank Pilsung because he caused me to rethink the whole C load 31x10.5 vs the P metric 265/15R15 mentioned above. I know this isn't conventional wisdom so many will dismiss what I'm about to say as heresy but...
Fact is that a P-metric tire will have a greater rated load carrying capacity at lower pressures even when the 90% reduction is accounted for. Using the Cooper AT3 as an example:
-265/75R15 load capacity is 2469 lbs. @ 35 psi (per P metric rating criteria) 2469*0.9=2222.1 @ 35 psi
-31x10.5R15 load capacity is 2270 lbs @ 50 psi (per LT rating criteria) as you reduce pressure load carrying capacity diminishes
More on inflation tables following this:
Fact is I don't hear many folks discussing running an LT or flotation tire @ 50 psi.
Calculating the proper pressure to run at, for a given load, is no trivial task as tables are hard to find. The reality is that as you drop your pressure you are also dropping your effective load capacity.
I'd wager that most folks run highway pressures of about 32 psi (give or take a few psi). So, when using a P-metric tire you are running "relatively" close, but slightly below, the advertised load capacity (this obviously varies by tire and inflation tables can be tough to find, I included a link to a Toyo tires chart below). In an LT tire, however, @ 32 psi you are well below the requisite pressure for the advertised load capacity. From my research the reduction in load capacity is non-linear so one should not assume that dropping pressure to 25 psi will decrease load capacity by half. So, what this means is, @ ~32 psi, give or take, you are running "under inflated" which has consequences for heat generation and tire failure on the highway. Of all things I try to avoid while driving, highway tire failure ranks near the top of the list. I read/hear of lots of folks running flotation/LT metric tires in the mid 20s to offset the negative ride characteristics of those "stiffer" tires. This, for most highway driving, is technically unsafe.
Nitto publication on load capacities and heat generation of an LT metric tire at 30 psi
http://www.nittotire.com/Content/pdf/Replacing Tires on Light Trucks - Technical Bulletin.pdf
Link on this page will get a pdf with load & inflation tables
http://toyotires.com/tires-101/load-and-inflation-tables
From the above linked publication:
A Toyo 265/75R15 is good for 2392 lbs @ 32 psi. 2392*0.9= 2152 at 32 psi
A Toyo 31/10.5 is good for only 1595 lbs @ 30 psi and 1785 @ 35 psi. 1945 @ 40 psi, 2100 @ 45 psi, 2270 @ 50 psi
A Toyo LT 265/75R15 is good for only 1750 lbs @ 35 psi, 1925 at 40 psi, 2090 @ 45 psi, 2270 @ 50 psi
Of course all of this ONLY matters if you are pushing the load capactiy of the tires to begin with. Which, would be an argument against P metric tires? BUT, as this example illustrates you will NEED to run much higher psi on the freeway to achieve good load carrying capacity on an LT tire.
Maybe there is a real good reason all those engineers employed by the vehicle manufacturers recommend P metric tires for vehicles as "robust" as the Montero and Land Cruisers?
Furthermore, why do we air down for trail use? The answer is to deform the tire carcass and "wrap" irregular surfaces for the sake of traction. A p metric tire having a softer, more supple carcass than an LT tire will (likely) deform more at a given inflation pressure. It logically follows then, that a softer p metric tire can achieve the requisite deformation at a higher pressure while maintaining a higher load capacity. Of course this higher pressure may make the tire more susceptible to puncture. The flip side of that is the tire may be less susceptible to losing a bead. I'd argue that a puncture, within reason, is an easier (safer) field repair than reseating a bead.
For a somewhat detailed discussion of the inflation curves of p metric vs. LT tires see this thread (many links are dated but some google-**** will yield good results):
http://forum.ih8mud.com/80-series-tech/111671-tire-inflation-ultimate-answer.html
Now, the easy counter argument is that a "C, D or E" load tire is needed for more "durability" in a harsh environment. Many will argue this based on the "number of plies" required to achieve the alpha rating. The reality is that the alpha rating has long been divorced from the "number of plies" comparison and most modern tires are constructed of 2 sidewall plies and 3 tread plies. The primary difference is the structure of those plies with the higer alpha rated tires having plies that can withstand the increased inflation pressures required to achieve the "load rating". I'd ask, however, who really runs a load C tire at 50 psi or a load E tire at 80 psi!!?
At any rate, significant anecdotal evidence suggests that a higher alpha rated tire will sustain more abuse on the trail. I'm not going to argue against that...there are many, many people with far more trail miles under their belts who have made that assertion and the masses are most likely correct.
However, for me, and likely many others the following consideration is really nagging at me:
I drive MORE miles on the freeway than the trail. I drive MORE fast on the freeway than the trail. I'm actually MORE concerned about tire failure on the freeway than on the trail. Tire failure on the trail would not be fun and may cause some serious delay in your adventure. Tire failure on the freeway may result in death. Do I really want to run a tire on the freeway that requires nearly 40+ psi to achieve the necessary load rating?
OK, geek out over...and I'm donning the flame suit because I imagine a lot of folks are running flotation type tires and the load range of those is significantly reduced under 40 psi
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