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Tire pressure question

30K views 32 replies 16 participants last post by  MagsCruiser 
#1 ·
Have the MT BAJA ATZ P3 285/70/17. Tire calls for 80 psi. Seems absurd to me. Any thoughts on what psi I should run? Any one have driving experience with them and at what psi? Thanks
 
#2 ·
Damn I couldn't imagine 80psi in the tires. I had the BFG KOs and at 55psi the rear and front wanted to swap ends while beating me to death. I ran those at 32psi and that worked pretty good.
 
#3 ·
Unless you're towing 8000lbs of gold bullion back there, there's no way 80 psi is appropriate. The label on the side of the tire list the MAX pressure, not the recommended pressure for this vehicle.

You'll find the recommended pressure located on a sticker inside the driver's door frame. It will likely say 32 psi. I would go to 34 psi and test it out. If your vehicle skitters all around the road at highway speed, drop the psi a little.
 
#5 ·
The LT tire w/10 ply rating does spec 80 psi as max pressure. 80 psi on the FJC and the ride will feel terrible. I run 10 ply 285/75/17's at 45 psi on the highway and am satisfied with the ride. There are no hard rules for LT tires, but I would not recommend road pressures below 30 psi.

Hope this helps.
 
#7 · (Edited)
I run 10 ply 285/75/17's at 45 psi on the highway and am satisfied with the ride.
Just to clarify, there are no "10 ply" LT tires, only "10 ply ratings", an archaic rating system carried over from the days when bias-ply tires existed.

Typical "10 ply rated" LT tires have 2, rarely 3 actual plies in the sidewall and 3 or sometimes 4 plies under the tread.
 
#6 ·
Start at 35PSI. If the ride is too rough for your driving conditions, go to 32PSI. If it's too soft for corners, go to 38PSI. If you have a lot of weight at the front of the truck, maybe stagger the PSI. I run 38PSI in the front and 35PSI in the rear and it handles amazing on and off road.

Whatever you do, don't inflate them to 80PSI...
 
#8 ·
Would the chalk test be a good start for that size of tire?
I ask because I've been tinkering with tire pressure this past week and a half with my 285/70/17 Cooper STT Pros because even at 32psi, the only contact they made with the road was the centre 6 inches.

After the chalk test, I was down to 27 psi and still had a strip of chalk on both edges. Not satisfied, I decided to do more research because I didn't want a reverse mohawk on my tires. My research, ironically enough lead back to a thread on our forum, to a thread called the 4 PSI Rule: http://www.fjcruiserforums.com/forums/wheels-tires/250106-4-psi-rule-3.html#/topics/250106?_k=uadhm1

So I started trying it a few days ago. I'm not sure if it's 'the answer' but it seems to make sense..but who knows.

The only issue I'm concerned about is that I read somewhere that a 10 ply tire is not meant to have less than a certain amount of air pressure( I can't remember the number ), which I'm well below, and I'm technically not low enough going by the 4 PSI rule..I'm seeing a 2.5 psi rise after 1 hour, starting at 25 psi cold.

The ride and handling actually improved at this level. I haven't done a mileage test yet so that's a mystery. Tire wear isn't noticeable at this point, and my contact patch is where it should be.

Time will tell how this test goes. I'm certainly not an expert on the subject but i want to figure this tire pressure mystery for oversize tires. Opinions seem to be all over the place in what's right, and they all make sense in their own way, adding to the confusion.

What do you think? Does this method make some sense to some of you?


Sent from a NiPhone, using auto-correct to eff things up with Tapatalk
 
#9 ·
Would the chalk test be a good start for that size of tire?
All comments are predicated by "IMO", right? :wink

The chalk test is tainted from the onset when using it to validate the correct air pressure of LT tires running on an FJC. By design, the load rating is such that the FJC does not sufficiently load the shoulder and tread area. To achieve the contact goal per the chaulk test, as you point out the tire for practical road use becomes underinflated.

My approach has always been (and see rule at the top) to find the balance between hardness and comfort of ride. I mentioned earlier I run 45psi. This gives me a good handling and road/ride feel and an advantage for achieving slightly improved fuel mileage. The more air pressure in the tires, the less tread surface in contact with the road. The less tread in contact with the road equates to less friction, less thermal rise, and less exercise chasing the magic 4psi rule. BTW - a good suspension system helps make a difference.

The fallacy in the 4psi rule is again the fact that the LT tires designed for heavier payload than the FJC, therefore the result of the experiment is less psi than what is optimum to achieve fuel economy and minimized wear. Will the LT tires wear in the center when overinflated? I guess this would be a valid argument, but I dont think this shortens the overall life of the LT tires.

In the end, our goals are to make the tires safe, comfortable, and without daily maintenance. Its a lot of subjectivity, but a lot of common sense too.

Cheers.
 
#20 ·
I've never had anywhere near 40psi in my tires on a vehicle this size. My brother's Raptor need requires around that amount of pressure, and it's huge next to the FJC, and weighs significantly more, so using that much air pressure makes no sense to me..but maybe I'm missing something here??

The most I tried was 35 psi, which was great all around. I read of several cases here on our forum of people having scary experiences with the ABS/Brake Assist function on our FJCs when slowing down on a bumpy road where the brakes would come on full bore and if I remember correctly, it was due to tire over-inflation messing with the system..but I'm going by memory here. I remember trying my best to recreate what people were describing, but I could never make my FJ do it.

I went to our local tire shop where I bought my Coopers to talk to Chris, the owner and also a good friend of mine, because I wanted to order a fifth STT for the spare, and a matching zero offset rim to match the other 4. I thought I would ask his opinion. I explained the different ideas being thrown around. He agreed that an E tire is meant to have a certain amount of psi, to match the minimum load rating for an E tire. He also said that he's seen them filled to 80psi, towing a 5th wheel horse trailer, where the sidewalls were noticeably squished down, driving long distances without issues or failure. After he added, that my FJC will never load the tires to the point where the sidewalls are that squished down, so running lower than the minimum pressure for an E tire on such a light vehicle should be fine, and that a bigger tire also requires less air pressure. Those minimal rating were developed when tires were bias ply, and todays tires are much more durable than when those standards were developed because the leap in tire tech. I agree with him. It's more food for thought in helping you come to your own conclusion. I'm still up in the air as to what's actually right, and will keep experimenting for now while keeping a close eye on tire wear.

So far, the ride/handling are quite outstanding where I'm at, I just have to see how things are wearing. I have no armour on my FJC, so it's close to stock weight, just a bunch of lights and the odd fat friend>:D
 
#21 ·
The most I tried was 35 psi, which was great all around. I read of several cases here on our forum of people having scary experiences with the ABS/Brake Assist function on our FJCs when slowing down on a bumpy road where the brakes would come on full bore and if I remember correctly, it was due to tire over-inflation messing with the system..but I'm going by memory here. I remember trying my best to recreate what people were describing, but I could never make my FJ do it.
This happens to me sometimes when coming down my driveway too fast (It's a rough dirt road). Basically, if you are trying to slow down while hitting serious bumps (the kind that would wreck most vehicles), the brakes will detect that you aren't slowly down properly and go into panic mode. I suspect that the bumps are also changing your pressure on the brake pedal so it further confuses the braking computer. So, it's not purely related to tire inflation but, I'm sure that over inflated tires would greatly exacerbate it just because they will make the bumps a lot more jarring.
 
#22 ·
I have found that D sidewalls are more in the load range of the FJ. LT and E sidewalls have a far greater load range and make for difficult PSI tuning on road and offroad.

I street at 40 PSI, net 48K miles from 285/70/17 D Wrangler Duratracs, finishing my second set now.
Trail at 20 PSI
Sand dunes at 14 PSI, netting a larger patch than what I could get from E sidewalls @ 8 PSI.

40 PSI on D is plush compared to 40 PSI on E or LT.
 
#27 ·
No argument that the D rated tire will provide a more comfortable street ride, and probably a more cushioned trail ride too. Running the LT tires is a compromise in road comfort. The primary reason for going with the E rated tire is or course trail use. One of my fears is a sidewall puncture while on rocky trails so I hedge against this by running the LT tires.
 
#23 ·
Yeah going over bumps fast or uneven pavement while braking hard can cause strange things to happen. Basically the tire looses traction (hops) and the brake is able to lock up that tire for a fraction of a second, then ABS kicks in thinking its doing the right thing and releases that brake and causes your vehicle to act strangely and your not able to stop quickly. Happened to me once while stopping on uneven pavement but I was barely able to notice it happening. Just felt it in the pedal. Having a high tire pressure will make the situation worse.

Same reason why stopping fast on gravel sucks in Toyotas. ABS kicks in and its like your driving on ice. The best way to stop the ABS intervention is to release the brake pedal (may seem counterintuitive) Then get back on it to slow yourself down.
 
#24 ·
I tried my hardest to make mine respond like that when I brake over big bumps but still can't get my FJ to go into panic mode. I'm glad though lol!

I settled on 27 psi for my STT Pros. This seems to be the happy spot for my FJ from what I can see and feel so far. I don't have metal bumpers or aftermarket skids, so I'm not carrying any extra sprung weight. Mileage hasn't taken a hit, it rides and corners better than ever( better than the P tires did at 35 psi ), and is much less twitchy on the highway..the highway ride has me mind-boggled. Even my brother who drives a supercharged Range Rover was impressed on the highway.

I'm still going to keep a close eye on tire wear though.


Sent from a NiPhone, using auto-correct to eff things up with Tapatalk
 
#25 ·
I tried my hardest to make mine respond like that when I brake over big bumps but still can't get my FJ to go into panic mode. I'm glad though lol!
I can't reproduce it on demand either. It's very, very noticeable when it happens because a slight increase in pressure on the brake pedal results in the exact same effect as slamming it to the floor. You have to completely release the brake pedal before it leaves panic mode.
 
#28 ·
Hiiiiiiiiiii from northern mass. just going to through my 2 cents in. I just ordered a set of 4 Mickey Thompson 295/70/17's. The tire guys say to start with 40-45psi and see how the vehicle rides and the tire wears. Meeeeeeeeeee the first test drive would be the key. Then adjust down? Mytires should be on late next week. I'll let u know how things go. Love the hybrid design. :)
 
#30 ·
I just bought my second set of E rated Duratracs. No regrets. Over 60k miles on the first set with a bit more mileage left yet... but with our winters I don't take too many chances.

The tough tires have saved me from walking many times. I've sliced and diced them and they still held up.

:cheers:
 
#31 ·
Here is a formula RVr's use for tow vehicles that requires weighing each axle. This is nice as manufactures list tire pressures for max vehicle load on stock tires, which beats the crap out of you if you are empty. For instance my Superdudy is supposed to be 70 front, 80 rear. This formula puts me around 60 lbs F/R when I have 400 lbs of tongue weight . I run 65 lbs for a little safety margin. My tires are wearing good and the ride is now tolerable.

Formula: ( Axle Weight / # Tires ) / Tire Rating ) x Max. Pressure Rating = Recommended Tire Pressure .
This works real good as is takes in to account your ACTUAL tire load rating of the tire and the load you are ACTUALLY carrying.

*note- tire rating is max tire carrying capacity listed on sidewall at MAX tire pressure for tire.
 
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