Calculate Tire Pressure (1 Viewer)

[JBHorne]

After talking to a few guys about tire pressure, I was worried. Everyone seems to have a different opinion on tire pressures when converting from a p-metric tire to an LT tire. Many say you are fine to go by what is written on the door, which is not always correct.

Here is how I personally calculate tire pressure. You need to get the specs from YOUR tire on the sidewall.

The LC200 (5.7L) has a curb weight of 5,690lbs. We need to add some weight because we have 1-2 people, bumper(s), winch, sliders, heavier tires, wheels, skid plates, lights, roof rack, etc.

I don't want to have to calculate all of that, so lets just round up to 6,000 lbs.

6000/4 = 1500 lbs per tire.

The Nitto TG's in 275/70R18 are rated at 3640 lbs @ 80 psi.

If you take 1500/3640 = .412

That means that I am loading up 42% of what the tire can handle (these are load range E tires -- some of you only have D tires).

Now, if you take 42% of 80 PSI you get 33.6.

That means that I should be running NO LESS than ~34 PSI in each tire.

To play it safe, I will run 35PSI and give me some leeway.


I hope this is a "safe" way to perform these calculations, and be sure to let me know if I am wrong on anything.

I just don't want to see you guys running some 28 PSI on the highway like a few have mentioned (not on this forum).

Stay safe!

 

[chimyz]

Hi JB...I originally tried the "feel" method for airing up my tires and compared it to your math, and I'm not sure it works for me...

From the dump the other day, I got an unloaded weight of 6500 lbs. This included me, ARB Bumper, Slee Sliders & steel rims, but with thrid row seats out.

My tires are same as yours...rated for 3640, but I used a calculation of (7000 lbs / 4 wheels) = 1750lbs per wheel.

1750/3640 = .48

Rounded up to 50%

50% of 80 = 40


To get to the point...

When I got the tires mounted, they aired them to 60 psi cold. It felt like they were too hard. I got my sensors updated with the rig, and it read around 60 - 65 on the dash.

I aired down to 40 and thought they were ok, but too mushy. It was confirmed when I loaded up about 500 - 600 lbs (up to the 7000 lb total) and the sidewall was 1/2 of what it should be. They darn near looked flat.

Immediately after loading it up, I went to the gas station and filled it up to 55. This was a wee bit too much after a little drive and aired them back down to 50.

So far 50 psi feels right for the road.

These numbers only apply to E rated tires with 3640 lbs. Anyone running the D rated tires and carrying a ton of weight may want to carefully watch what pressure they are running. Don't want to have the whole Firestone/Explorer incidents happening, and all kinds of LC200s are rolling due to tire explosions.

 

[JBHorne]

Hi JB...I originally tried the "feel" method for airing up my tires and compared it to your math, and I'm not sure it works for me...

From the dump the other day, I got an unloaded weight of 6500 lbs. This included me, ARB Bumper, Slee Sliders & steel rims, but with thrid row seats out.

My tires are same as yours...rated for 3640, but I used a calculation of (7000 lbs / 4 wheels) = 1750lbs per wheel.

1750/3640 = .48

Rounded up to 50%

50% of 80 = 40


To get to the point...

When I got the tires mounted, they aired them to 60 psi cold. It felt like they were too hard. I got my sensors updated with the rig, and it read around 60 - 65 on the dash.

I aired down to 40 and thought they were ok, but too mushy. It was confirmed when I loaded up about 500 - 600 lbs (up to the 7000 lb total) and the sidewall was 1/2 of what it should be. They darn near looked flat.

Immediately after loading it up, I went to the gas station and filled it up to 55. This was a wee bit too much after a little drive and aired them back down to 50.

So far 50 psi feels right for the road.

These numbers only apply to E rated tires with 3640 lbs. Anyone running the D rated tires and carrying a ton of weight may want to carefully watch what pressure they are running. Don't want to have the whole Firestone/Explorer incidents happening, and all kinds of LC200s are rolling due to tire explosions.

Hmm, good point then. I find it interesting that I cannot find these calculations anywhere! There HAS to be some formula that tire manufactures use....

 

[chimyz]

Hmm, good point then. I find it interesting that I cannot find these calculations anywhere! There HAS to be some formula that tire manufactures use....

I always thought the 3640lbs @ 80psi was to not go over that weight/tire, or go above the psi amount...I didn't even think about the minimum. I've always treated tire pressure as a feel thing, and I've always been able to tell when there is too much or too little.

 

[dtt255]

Wow..we got mathematicians!

 

[chimyz]

Wow..we got mathematicians!

Just JB. I'm a feelatition.

 

[dtt255]

Hi JB...I originally tried the "feel" method for airing up my tires and compared it to your math, and I'm not sure it works for me...

From the dump the other day, I got an unloaded weight of 6500 lbs. This included me, ARB Bumper, Slee Sliders & steel rims, but with thrid row seats out.

My tires are same as yours...rated for 3640, but I used a calculation of (7000 lbs / 4 wheels) = 1750lbs per wheel.

1750/3640 = .48

Rounded up to 50%

50% of 80 = 40


To get to the point...

When I got the tires mounted, they aired them to 60 psi cold. It felt like they were too hard. I got my sensors updated with the rig, and it read around 60 - 65 on the dash.

I aired down to 40 and thought they were ok, but too mushy. It was confirmed when I loaded up about 500 - 600 lbs (up to the 7000 lb total) and the sidewall was 1/2 of what it should be. They darn near looked flat.

Immediately after loading it up, I went to the gas station and filled it up to 55. This was a wee bit too much after a little drive and aired them back down to 50.

So far 50 psi feels right for the road.

These numbers only apply to E rated tires with 3640 lbs. Anyone running the D rated tires and carrying a ton of weight may want to carefully watch what pressure they are running. Don't want to have the whole Firestone/Explorer incidents happening, and all kinds of LC200s are rolling due to tire explosions.

So you keep your tires at 50 psi for normal daily driving?

 

[chimyz]

So you keep your tires at 50 psi for normal daily driving?

Yes, I've got them at 50 psi right now, and the bottom of the tire is slightly compressed.
The calculation above would only apply to the E rated LT type tire JB or I run, different calculations and PSI for different tires.
Your's would work like this:

Let's say your LC is 7000lbs.

Your specific tire reads:
285/60/18 3086lbs @ 50psi max
(non LT, but extra load, so no "D" or "E" rating)

7000 / 4 = 1750
1750/3086 = 0.567
56.7% of 50 = 28.35 psi

So based upon JB's math, the minimum you could run safely on street would be 28.35 psi. That sounds about right...I had those tires originally, and I ran them cold at 33 psi, and they were great in pretty much all road conditions.

 

[donjuanmiguel]

This forum is too cool! After reading the thread I calculated where I should be. Turns out I've been running about 10psi to low. JB, you posted alignment comments on another thread, I'll be heading that way in a bit with what I've discovered about mine.

 

[JBHorne]

I should add that this math is just what I came up with in my head and therefore I hold no liability for any injuries that may be sustained by inflating to such pressures.

Use this information at your own risk. For me, it works. Like Chimyz said, though, it really comes down to a question of feel. Safety is paramount.

And no, I hate math! Just sometimes it helps.