Recommended tire PSI for MTs on street?

[eddiebx]

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.





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.

I think the big difference here is you have massive LT tires, lol. You're talking about those 38" tires listed in your sig right? I think you're not exactly comparing apples to apples. The actual volume of air plays a huge role in "load carrying capacity per PSI", so to speak...and clearly a 38" tire is going to hold waaaaay more air than a 32-33"

For the same exact size tire, the LT tire would require more pressure than the P-metric, is how I understand it.

For example, on my 5th gen I went from stock 265/70R17 to LT 265/70R17 (GY duratrac, in case you're curious) and I put in ~45psi. I monitored that for 7k miles of mix driving and the wear came out pretty much even from a practical stand point (front shows ~1/32" more shoulder wear, rear shows ~1/32" more center wear; which theoretically should all balance out after rotation). This was the recommended pressure listed on my receipt from Discount Tires, and also the same pressure that I extrapolated from the TRA's tables.

I truly don't mean any disrespect, but I don't think there's really a "rule of thumb PSI" when it comes to LT tire pressure because so many variables in tire sizes and what not. IMO, load charts are kinda essential here when trying to convert from one to the other safely.

 

[JB.]

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.

Well, the tag line is "to match the load capacity of a P type tire of the same dimension".

It would only impact you if you wanted to match the OEM load reserve established by the stock tire. I wouldn't be surprised if the road pressures you run put you in that ballpark anyway, but I don't know what tire you're talking about.

 

[jsmith21]

I'm running 38psi in mine right now, and they're doing good. Ride isn't bad, firm, but it's a truck so I like that. Will probably lower the pressure when it warms up though. 255/75/17 Goodyear MTR's.

 

[BlackWorksInc]

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?

Factory FJ TRD is LT265/75R16 iirc or LT265/70R17. I am running LT285/70R17 Nitto Terragrapplers on stock Sport Edition rims.

 

[JB.]

Will probably lower the pressure when it warms up though.

Don't know what your goal is but if a change is made for outdoor temperature, lower would generally be the opposite direction to go for hot weather.

 

[jsmith21]

I thought that as air heats up, it expands. Therefore, as the summers in the south get pretty warm, the air pressure in the tire would naturally go up as the temperature rises, and would rise even more with driving, would it not? I know from my experience racing that on a summer day we run a lower tire pressure than we would on a cold day, to compensate for the extra pressure that will build up because of the extra heat, and ensure an optimum contact patch.

 

[JB.]

Factory FJ TRD is LT265/75R16 iirc or LT265/70R17. I am running LT285/70R17 Nitto Terragrapplers on stock Sport Edition rims.

Apologies if I'm repeating myself, but the 46psi equivalent for your tire is 40psi, so you could run that and still be in full "compliance".

I thought that as air heats up, it expands. Therefore, as the summers in the south get pretty warm, the air pressure in the tire would naturally go up as the temperature rises, and would rise even more with driving, would it not? I know from my experience racing that on a summer day we run a lower tire pressure than we would on a cold day, to compensate for the extra pressure that will build up because of the extra heat, and ensure an optimum contact patch.

The "cold" reading automatically adjusts for the season unless a climate controlled garage or something throws a wrench into it.

Assuming the cold reading comes from temps close to outdoor temps, then the tires have already been adjusted for the season if you're maintaining a consistent target pressure. Maybe that's what you meant. If so, apologies.

The tire will run cooler in hot weather with more pressure, though, if that's what's desired.

 

[nevada]

I think the big difference here is you have massive LT tires, lol. You're talking about those 38" tires listed in your sig right? I think you're not exactly comparing apples to apples. The actual volume of air plays a huge role in "load carrying capacity per PSI", so to speak...and clearly a 38" tire is going to hold waaaaay more air than a 32-33"

For the same exact size tire, the LT tire would require more pressure than the P-metric, is how I understand it.

For example, on my 5th gen I went from stock 265/70R17 to LT 265/70R17 (GY duratrac, in case you're curious) and I put in ~45psi. I monitored that for 7k miles of mix driving and the wear came out pretty much even from a practical stand point (front shows ~1/32" more shoulder wear, rear shows ~1/32" more center wear; which theoretically should all balance out after rotation). This was the recommended pressure listed on my receipt from Discount Tires, and also the same pressure that I extrapolated from the TRA's tables.

I truly don't mean any disrespect, but I don't think there's really a "rule of thumb PSI" when it comes to LT tire pressure because so many variables in tire sizes and what not. IMO, load charts are kinda essential here when trying to convert from one to the other safely.

I was talking about my bfg's actually. they are 35's, so still pretty big. but really I was just pointing out that they max out at the same pressure as a Pmetric tire, so you cant "go higher". altho, I don't know that ive ever seen a p tire that large. I have seen a set of 32's. they were mud tires even. very odd.

Well, the tag line is "to match the load capacity of a P type tire of the same dimension".

It would only impact you if you wanted to match the OEM load reserve established by the stock tire. I wouldn't be surprised if the road pressures you run put you in that ballpark anyway, but I don't know what tire you're talking about.

I guess that's kind of where im not understanding things. according to the "spec" pressures, the load rating is way above the GVWR. I understand a built in safety zone, but an additional 5,000lbs? that's virtually double what is needed, that seems a bit excessive.

Factory FJ TRD is LT265/75R16 iirc or LT265/70R17. I am running LT285/70R17 Nitto Terragrapplers on stock Sport Edition rims.

those are 10 plys then?

Apologies if I'm repeating myself, but the 46psi equivalent for your tire is 40psi, so you could run that and still be in full "compliance".

I agree. 40 is much closer to where it should be.

 

[JB.]

I was talking about my bfg's actually. they are 35's, so still pretty big. but really I was just pointing out that they max out at the same pressure as a Pmetric tire, so you cant "go higher"

Right, but you only need to go higher in pressure if the size hasn't changed. If you go up in size a little bit, then you only go up in pressure a little bit. Go up in size enough and you may actually have to "go lower" in order to match the stock P tire.

Using 35x12.5-15LT as an example (which is what it sounds to me like you have) and using a 4th gen for computation, it would take about 30psi or a little less to match stock load reserve (couldn't find that exact size in a load table, but it's a close estimate). Obviously, it would take even less on the '85.

 

[eddiebx]

I guess that's kind of where im not understanding things. according to the "spec" pressures, the load rating is way above the GVWR. I understand a built in safety zone, but an additional 5,000lbs? that's virtually double what is needed, that seems a bit excessive.

Oh okay, I think I finally see what you are saying! I don't think manufacturers pick the recommend tire pressure solely for the purpose of supporting the maximum load capacity. I think they go through a lot of testing to determine the best pressure to set the OEM tire at to optimize overall handling, fuel efficiency, wear, etc...and it just so happens to end up way over the GWVR. (ie. sidewall rolling over too much during cornering, too much rolling resistance killing MPG; bump up the pressure to resolve even though the tire can safely carry that much load already at whatever pressure)

I guess that's why looking at all this based on the load capacity alone can be kind of misleading...the problem is when you start changing tires outside of OEM spec, the load rating is the only thing "standard" we have to go by when trying to match the oem specifications as closely as possible.

The correct answer is probably test, test, and test your new tires to come up with the new optimum pressure (like how a car manufacturer would test OEM tires); but what kind of hobbyist has those means, lol.

 

[Ohcrymore]

Thanks, but that thread got me even more confused.

I found two things online a tire pressure calculator equation and the chalk test.

But after the calculation, it said I should run at around 25psi, that seems a bit low, so I'll just try the chalk test this weekend to see what results I get.

And for the record, here's the equation I found, not really sure if it's accurate or not, but I kind of understand the reasoning behind it. If anyone care's to chime in, please do!

Got it from some heep forum



The curb weight of an 08 SR5 4x4 v6 is 4300lbs+500lbs (random arbitrary weight I added to account for car mods heavier than OEM, random stuff in the car, weight of driver, and an additional passenger), total weight of 4800lbs.

The tire I got has a max PSI of 65 @ 3195 lbs.

So the calculations would be:

1: 3195*4=12780lbs

2: 4800lbs/12780lbs=~38%, rounding up to 40%

3: 65PSI*40%=26psi?

So according to this "calculation" it says to run 26psi, that does not sound right at all. maybe I did something wrong there? anyone care to chime in?

At this point, I'm more confused than I was before, so I'll be doing the chalk test to see what that gets me. Ironically, the YouTube video describing the chalk test is from another Heep driver

4800 lbs? My gross weight on the door states 5800lbs...