nevada
Senior Member
MAYBE for a stock size tire. not for a 33.
Recommended tire PSI for MTs on street?
- Thread starter JustSomeName
- Start date
MAYBE for a stock size tire. not for a 33.
JustSomeName
New member
im not knocking your theory nor the other poster's theory, just trying to gather as much information as I can before I make a decision.
from your response, it seemed like you thought the poster suggested I ride at 80psi, but he actually recommended in the 40-45psi range, just thought id try to clarify
eddiebx
New member
I dunno what to say about you not trusting that chart...it's created by the TRA (Tire and Rim Association). If I understand correctly, they are the ones that write the standards here in the US.
Not really sure how the indicated weights correlate to the real life GVWR (maybe it's per axle, per tire...no idea), but the theory behind what I was trying to demonstrate is how to match the air pressures to make sure the LT tire can safely carry at least as much weight as the P-metric tire you are replacing.
What you said earlier about a larger tire needing less air is entirely true. The very charts that you don't trust, prove you right, lol.
Anyways, I suggest you give that chart a read, and the examples in it, I think it will make a lot more sense if you sit down and compare their examples step by step
BlackWorksInc
New member
I don't quite understand your reasoning here, you're logic seems off. Tire diameter is only a small part of the issue. We're talking about compressible gas here, volume is affected by pressure and the container it fits in. For example (we're going to simplify this and assume that all aspects of the following hypothetical tires is identical besides diameter); a tire that is 32" @ 35psi is going to have less physical air in it than a tire that is 33" @ 35psi because there is more internal volume.
Of course we've already determined from multiple sources that an LT tire requires a higher pressure to maintain a similar load rating to a P rated tire, which means that more physical air is required to properly maintain the tire's ability to hold weight and more importantly decrease rolling resistance (notice many LT charts start AFTER 35psi, many of us speculate this has more to do with heat build-up, deflection, and flexing of the sidewall than load carrying ability), thus yes you COULD run the tire at 30 or even 25psi as many theorize, but the reality is that there would probably be more wear on the tire beyond simple erosion of tire material.
One of the issues is that we need to realize that tires are constructed differently from each other in respect to type and this is why you have a 32" tire that needs 32psi to hold a 4th gen up, but a farm tractor only needs like 15psi to hold up a 6ft diameter tire.
I'm still looking into this as it is weirdly confusing and convoluted. :nicethread: I'd love to hear from [MENTION=2798]JB.[/MENTION] on the topic as I am barely keeping up with the amount of information in the thread, the links, and what I have been Googling.
JB.
New member
JustSomeName,
It sounds like you must have LT285/70R17. For reference, to equate to the 46psi Toyota psi Blackworks mentioned, that tire would need about 38psi -- since the tire is larger --, but that would represent the high side of where you should start. See my reply to [MENTION=66389]BlackWorksInc[/MENTION] for what that reference point means.
Nothing can replace your experience and how the tire wears on your truck with your driving. I'd just put 'em around 35 and see how they feel and wear, then adjust accordingly.
It sounds like you must have LT285/70R17. For reference, to equate to the 46psi Toyota psi Blackworks mentioned, that tire would need about 38psi -- since the tire is larger --, but that would represent the high side of where you should start. See my reply to [MENTION=66389]BlackWorksInc[/MENTION] for what that reference point means.
Nothing can replace your experience and how the tire wears on your truck with your driving. I'd just put 'em around 35 and see how they feel and wear, then adjust accordingly.
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JB.
New member
Load capacity is simply a function of size and psi within tire type, and there's probably less precision involved than you may assume.
The basic message is that LTs carry less load than a P type tire with the same dimensions and psi.
The question then becomes what, if anything, should be done with that information.
Tires can be run at a lot of different pressures for a lot of different reasons, but there are two widely referenced target road pressures that each have legitimacy but are quite far apart from each other, making everything look confusing when the two PSIs are compared to each other. They are:
(1) Tire pressure based on load capacity as listed in load tables. It's a roll your own pressure calculation based on the gross weight (or GAWR) of vehicle the tire is being used on. If no load reserve is added, it results in a much lower psi number than the one produced by the below.
(2) Tire pressure specified by the vehicle engineers for a specific tire/vehicle combo (or the load reserve equivalent for a different tire). It varies from vehicle to vehicle and includes many different considerations but it always involves, especially these days, a lot of load reserve, and part of the reason for a lot of load reserve comes from what has been learned about statistical tire safety in the years since Ford/Firestone.
The 46psi you referenced is mostly based on making sure there is no reduction in load reserve compared to the stock P-metric tire. I suspect and would be willing to bet that the pressure is more a response to the regulatory environment than it is a new number worked up from scratch by Toyota vehicle engineers. Since the LT tire now has as much load reserve as the stock P type tire and needs extra pressure in order to have it, the result is a psi number that is much higher than one would get simply calculating from a load table. Same thing with the stock P-metric tire but people rarely consult a load table for those. The industry standard for minimum tire load reserve (using GVWR or GAWR) seems to be 15%. Most cars and trucks are comfortably higher than that. The 4Runner is one of the highest which only makes the difference between (1) and (2) bigger. Long ago, I made a chart showing that in this thread.
What pressure Toyota vehicle engineers would choose if 4Runners were designed exclusively with LT265/70/17 tires would make for an interesting question. If I'm allowed to speculate, I suspect it would fall somewhere between 35 and 40psi.
There is no perfect answer for correct air pressure but for anyone who's not a tire nerd and wants a simple, single answer, (2) is a better place to start than (1).
I know the topic is convoluted. If I failed to address anything, please let me know.
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BlackWorksInc
New member
[MENTION=2798]JB.[/MENTION] Thanks, that actually helps a little bit.
Though its curious you mention a theoretical 4Runner having 35-40psi in "stock" LT tires, since the FJ Cruiser is slightly lighter and they call for 46psi. I just wish there were an easier way to calculate it. It seems that everything kind of works out okay up with stock tire sizes, but there's no real guide on how to adjust when you are going up in diameter but retaining the same tire type. i.e. how much different is a 33" LT tire @ 40psi versus a 35" LT tire at 40psi in terms of deflection of tire and ride characteristics and not just load capacity.
Though its curious you mention a theoretical 4Runner having 35-40psi in "stock" LT tires, since the FJ Cruiser is slightly lighter and they call for 46psi. I just wish there were an easier way to calculate it. It seems that everything kind of works out okay up with stock tire sizes, but there's no real guide on how to adjust when you are going up in diameter but retaining the same tire type. i.e. how much different is a 33" LT tire @ 40psi versus a 35" LT tire at 40psi in terms of deflection of tire and ride characteristics and not just load capacity.
Brutalguyracing
New member
The only sure wAy to tell is by doing the chalk test
Brutalguyracing
New member
The only sure way to figure out what tire pressure your own tires need on your vehicle, is to preform a chalk test period
Every truck is going to be different
BlackWorksInc
New member
Well I always park on an unpaved sidewalk behind the dealership that is just loose dirt and dust, which is pretty much the same as chalking up the tires...
So far my '04 V8 Sport with SW Front Bumper, 2.5" OLW Lift, CBI Sliders, and LT285/70R17 tires filled to 46psi seem to be contacting pretty evenly on the road. The chalk method is a good way to check, but I don't think its very accurate...
JB.
New member
I forgot you were talking about an FJ, but I stand by the hypothesis that much of it is regulation driven.
Deflection is tied directly to load capacity (within tire type) so that one's covered.
As far as ride characteristics, that's where the vehicle engineering comes in to it. If you change tires, you're kind of on your own but if you've established a good working psi for a particular LT tire then the logical place to start on a bigger one is at a psi that matches the load capacity of the former tire at the preferred psi.
BlackWorksInc
New member
Sorry to keep pestering you, but since we have you in the thread... I'd like to pick your brain on one more thing that I haven't found a clear answer on (and would like your opinion/second thought in regards to) and is slightly not related. GVWR is supposed to be roughly what the vehicle suspension and frame can safely and effectively support. So when we upgrade our vehicle's suspensions with heavier springs, beefier shocks, stronger components; doesn't this essentially increase our theoretical GVWR as we have essentially modified the vehicle's ability to handle such weight (I would assume our frames can take a bit more abuse than what the GVWR says it can because the weaker link here would be the suspension)? It sounds right in my head and from what I can fathom, but really I haven't seen much information that shows testing/proof?
Ol'Baditude
New member
This is true.
The thing with chalk tests is that it is very easy to see if you are 5-10lbs over inflated because you will see a 3/4-1" patch of chalk around the outer edge of the tire. However if you are 5-10 under inflated, you won't necessarily see a nice white line down the center line of the tire. It will more than likely appear as though you are getting nice even wear. Somehow, traveling slowly in a straight line just doesn't show you what you need to see, and running around with your tires 10lbs under inflated is just not good.
I've done this test from 30-45 psi in increments of 5lbs, and while 45psi showed clearly that the tire was convex and left a nice stripe of white on the outer edges, the 30 and 35 showed the same results.
Basically, what I did at that point was stop at the point where the wear was even all the way across(35lbs) and add a few psi for good measure and called it a day. The result was 38lbs and it produces a very smooth ride, even tire wear. And FWIW it also tests well with the 4psi rule which is nice for piece of mind.
nevada
Senior Member
i keep reading "LT tires need more pressure than P tires"
but, the LT tires on my 4runner cant follow that. they have a max pressure of 35psi.
that would give me 11,000ish lbs of load capacity. that is WAY to much.
I run them anywhere from 25-35 depending on what im doing. at 35, the ride is very harsh. that's because the tire is overinflated.
but, the LT tires on my 4runner cant follow that. they have a max pressure of 35psi.
that would give me 11,000ish lbs of load capacity. that is WAY to much.
I run them anywhere from 25-35 depending on what im doing. at 35, the ride is very harsh. that's because the tire is overinflated.
without looking at any tire charts, I would say your tires are overinflated. run them down to 40, I bet the ride quality increases, as well as its road manners. along with proper tire wear.
BlackWorksInc
New member
But that's the thing, they don't show signs of overinflation. The tread wear is pretty even across the tire (aside from some mildly quicker wear on the front's outside edge since I corner hard in my 4runner) and the ride isn't what I would describe as harsh, more firm (which I prefer in all my vehicles.)
I may play with the pressure in the future, like I said I just chose 46psi because its what the FJ Cruisers with the TRD wheels and LT tires are supposed to be running. So far I haven't had any ill effects as a result.
JB.
New member
I don't want to exceed my pay grade by answering, but sure. From a tire standpoint, GVW should be considered whatever maximum weight you plan on being able to carry.
nevada
Senior Member
but what size tires are on the FJ's? im guessing they are considerably smaller, and therefore would require more pressure. altho, I still think 46 psi is too high, even for the smaller/stock tire.
also, what exact tire are you running?
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