high pressure tires question

[Brian894x4]

OK, this might sound like a trivial question, but I have a question about tire air pressure.

All my prior tires, which I ran on my truck were Load Range C MTRs and BFG Muds and had maximum pressures at 33 and 35 inches of around 35 or so.

These new MTRs I have for the Land Cruiser are 285/75R-16s and I believe Load Range D, and have maximum pressure of about 66 psi.

I've been running pressures in the 30s, until I noticed what the sidewall said. Now I have them up to 50 psi.

My question is this. What pressure should I run these tires and why so much pressure difference in relatively the same size tire? I assume it's for the higher load capacity, but can these tires be run at lower pressure or do they need to aired up much higher the 30s?

I'm also curious what the difference is in the 35 and 66 psi tires. Are we talking different sidewall and will the high pressure tire react differently than the low pressure tire when being aired down to something like 10-15 psi?

 

[CodyLX450]

66 is MAX PSI.

You'd never need that much for an 80's weight.

Whatever PSI keeps the tire tall and not sagging down when on the pavement is good (probably 30-35 PSI)

 

[spressomon]

I ran my 285/75/16 GY MT/R's at 38psi for on-road on my '99 100 (bumpers, AO drawers, etc.). If your's is a stock rig then maybe 35pis... With additional load (i.e. camping, etc.) I would run 40psi.

 

[Desertdude]

Before I added all the weight to the 80 - I ran BFG KM 315/75/16 tires at 35 PSI everything was great solid ride with a bit of cushion - as the mods developed, and the 80 loaded down (close to 7K) I went 45- 50 psi - it seems to help with cornering and overall stability.

I switched brands to Open Country Toyos - (same size) they are 10 Ply Rated - little bit noisier than the BFG's but a much more stable ride with incredible grip, I run these at 45 psi

Wheeling moderate trails, tire pressure down to 25 psi - 20 psi in Moab - in the sand tires down to 10-15 psi

 

[Grim Reaper]

Tire trick that a Tire designer/tester put out at one timeon a Camper forum I am a member of that I found interesting. I have tried it and it seems to work quite well.

I worked for Michelin Tire Corporation for 7 years and Yokohama Tire Corporation for 11 years. I have given numerous tire seminars on tire maintenance and especially how to determine the correct tire pressures. So here goes.
The pressure on the sidewall of the tire is the maximum pressure at the published load at approximately 55 mph. (The speed can vary somewhat but it is not important for our discussion).
The air pressure is required to support the load that the tire must carry in such a manner that the tire flexes at the designed place on the sidewall of the tire.
If the load on the tire changes then the air pressure should change accordingly to keep the tire flexing at the proper place.
The reason for correct air pressure is to prevent the tire from overheating. It was put together with heat and it will come apart the same way. An under inflated tire will eventually self destruct due to excessive heat build up. An over inflated tire will ride harshly and be more likely to burst upon impact. Sorry for the long explanation but here is the bottom line.
To determine the correct air pressure, check the pressure when the tire is cold. Run the tire for several miles at highway speed. Stop and immediately check the air pressue in the tire. It should be higher than we cold but no more than 10% higher.
Now here is the hard to believe part. If the pressure is more than 10% higher you must ADD AIR and test again. For example if you start with 50 psi cold. If the pressure is 60 when hot, you have exceeded the (10%) in this case, 55 psi maximum safe heat build up pressure. You must ADD AIR. In this case I would add 5 psi which would take the tire to 65 psi when hot. After you run the tire again you will find the pressure to actually drop because the tire will run cooler. The heat build up causes the tire pressure to increase when under inflated.
On the other hand, if the 50 psi cold pressure does not change when hot. You have more air than needed. You can remove 5 psi or so and test again when they return to cold. Like the next trip you take.
So a fully loaded rig will require more air in the tires than one with empty tanks and a light load on board. Always error on the side of over inflation. Thus the maximum sidewall pressure indicated on the tire is usually used. It usually is more than needed. Each axle has its own requirement based upon the load on that axle.
So how do I get a 2 Rivet rating? Does this long explanation help or hurt?

 

[DaveInDenver]

Good post, Grim. Very helpful! I run my 33x9.50 BFG AT K/O tires at 35 psi and have found that my pressure doesn't vary much when I check it at the filling station. So I guess for a XtraCab truck with a 'Nest and 22R-E, 35 psi is a bit high for daily driving duties. The door panel says 29 psi with the stock tires, which is probably about right even for over sized tires. I have run them as high as 40 psi when loaded completely for a week long trip and I put 50 psi in them when I was doing landscape on the old house (had the bed filled to the rail with top soil, which in a 6' Hilux bed weighs 2000 lbs).

 

[paulj]

The 10% temperature change is new one to me, but makes sense. So with a cold psi of 35, after 20 minutes or so of driving, it should only go up to about 38. However such a test needs to take into account heating due to sun, and hot pavement.

Race car crews check the tread temperature after several laps, aiming for uniform temperatures across the width. Most other rules-of-thumb focus on contact area or tread wear.

paulj

 

[Desertdude]

Good info, I will have to test out this theory to see if I am close.

 

[kcowyo]

I run the BFG KO's in a 285/75/16, load range D, on the stock 7" wide rims.

I have always run them at 32-35PSI on the pavement. My Stauns are set at 18PSI. When I recently had the truck in for an alignment at a friend's Big O Tires store, he told me I was getting uneven wear from running them under-inflated . He said I should be running closer to 40PSI on the road, especially with the camper's weight on the rear.

He owns the store so I listen to him. I'm sure some of the uneven wear comes from having them on a 7" rim, when an 8" is recommended with 285's. Can't imagine the MTR's a whole lot different.

 

[adventureduo]

Brian, it's the higher load capactiy. That's all. You don't need to run 50 lbs in those tires unless you want them as hard as a rock.

I run 37lbs outback and 35lbs upfront. Right now, seems to work fine.

Yep, on Grims post.. the guy is correct.. heat can blow out your sidewall, that's why whenever i air down for a trail, i never do more than 40mph aired down.

 

[Brian894x4]

Yep. 50 psi makes them hard as rocks on the road. Absolutely horrible. Brought it down to just under 40. Much better. But I haven't done that test yet, mentioned above.

 

[crawler#976]

Brian, the 265/75-16 MT/R's on our '05 Taco are "E" rated, and currently have 30K on'm.

Initially they were run at 35 PSI, and after 5K I noticed the center of the tread was taking most of the wear. To keep them wearing relatively flat I've kept them at 30 PSI. Using the newpaper method to get edge to edge contact they needed to be at 26, but the MPG penalty was pretty severe.

To help soften the tires a bit and to improve rain/snow/ice traction we had them siped.

Mark

 

[ntsqd]

The 10% temperature change is new one to me, but makes sense. So with a cold psi of 35, after 20 minutes or so of driving, it should only go up to about 38. However such a test needs to take into account heating due to sun, and hot pavement.

Race car crews check the tread temperature after several laps, aiming for uniform temperatures across the width. Most other rules-of-thumb focus on contact area or tread wear.

paulj

Ditto, the 10% was new to me too. Interesting idea.

A note on the race car tire temp comment. You really only can see the last corner the car went thru. Straight line sections don't heat the tires enough to really matter. Depending on how out of square the chassis is (intentionally or otherwise) you may need to check temps on the exit of both lefts & rights.