Tire Pressures

[JBHorne]

For those of you with HEAVY 200's (arb bull bar, winch, skid plates, sliders, rear bumper, etc) with LT tires, what tire pressures are you running?

I know I should probably get the vehicle weighed and do the math, but just out of curiosity here. I currently run 50psi which I based on limiting the amount of bulging of the front tires. 45psi seems too low, but I think is more inline with the math.

 

[Willy beamin]

Over the summer I was running 33 psi and then this winter switched it up to 44psi. @gaijin seems to have knowledge on this subject so maybe he can chime

 

[Desert Nomad]

I think it even depends more on tire type, with a load range D tire you'll need to air up more to get rid of the bulge than on a load range E.

I'm currently running with a TJM bar, skid + trans plate, winch, custom steel rear wheel tire carrier and a steel ARB roof rack. Tires are Cooper Discoverer STT 33x12.5r17 load range D and keep the pressure around 38 psi, 42psi seemed best for me on road but I'll usually keep it at 35-38 if I'm going to be doing quick dashes through the desert without deflating.

 

[gaijin]

Over the summer I was running 33 psi and then this winter switched it up to 44psi. @gaijin seems to have knowledge on this subject so maybe he can chime

OK, but buckle up folks, it's going to be a bumpy ride ...

Without specific vehicle/tire info, all I can do is provide generalized info based on a stock 2013 LC200 running LT285/70-17 tires. It should be enough to get a feel for what may apply in your individual case.

As a base, we have the door sticker for the LC200:

The sticker gives us some important info:

- GVWR = 7,385 lbs
- GAWR Front = 3,595 lbs
- GAWR Rear = 4,300 lbs
- Tire size = P285/60R18
- Cold Tire Pressure = 33psi Front/Rear

From this info, using published Load Limit tables, we can calculate the Load Limit which results from the recommended tire size and pressure:

This calculation yields a Load Limit of 2,512 lbs (per P-Rated tire) @ 33psi.

Since we are talking about a LT-Rated tire replacing a P-Rated tire in our hypothetical, we need to calculate the required Load Limit for a LT-Rated tire. When replacing a P-Rated tire with a LT-Rated tire, the LT-Rated tire should have a Load Limit 10% less than the P-Rated tire. Therefore in our example, the LT-Rated Load Limit is calulated as follows:

P-Rated Load Limit / 1.1 (the 10% difference) = LT-Rated Load Limit
-or-
2,512 lbs per tire / 1.1 = 2,284 lbs per tire

Using this new LT-Rated Load Limit, again using published Load Limit tables, we can calculate the Cold Tire Pressure required to yield the required Load Limit. For the LT285/70-17 tires, it looks like this:

From the graph, we can see that approximately 39.3psi yields a Load Limit of 2,286 lbs which, when corrected for the P-Rated requirement = 2,515 lbs compared to the P-Rated requirement of 2,512 lbs. For simplicity, let's round up and call it 40psi.

It makes no difference whether the LT285/70-17 tire is D-Rated or E-Rated, the required pressure is the same.

**********
What we know now is:
For a stock LC200 running LT285/70-17 tires, the required Cold Tire Pressure is 40psi Front/Rear.
**********

Now, we can look at what might be appropriate for a LC200 which has additional weight added to it.

Relax, we've already slogged through the tough stuff - the rest is easy.

Examining the Load Limit graph for the LT-Rated tire above, we see the load Limits are approximately as follows at the indicated Cold Tire Pressures:

40psi = 2,315 lbs/tire -or- 9,260 lbs Total
45psi = 2,510 lbs/tire -or- 10,040 lbs Total -or- 780 lbs more than stock
50psi = 2,755 lbs/tire -or- 11,020 lbs Total -or- 1,760 lbs more than stock

So, roughly speaking:

- If you have added 780 lbs to your rig, you should be running 45psi instead of 40psi.
- If you have added 1,760 lbs to your rig, you should be running 50psi instead of 40psi.

But what if you have added a weight somewhere between the published 5psi increments?

It's not exact, but a simple calculation could be applied.

Let's say you've added 1,000 lbs to your rig.

Already you know you should be running somewhere between 45psi and 50psi. Between 45psi and 50psi, we see a difference of (11,020 lbs - 10,040 lbs = ) 980 lbs. 980 / 5 = 196 lbs per psi.

So, 1,000 lbs (desired increased capacity) - 780 lbs (added capacity @ 45psi) = 220 lbs / 196 (lbs/psi between 45psi and 50psi) = 1.12psi above 45psi = 46.12psi will yield a capacity 1,000 lbs above stock.

See? Easy, right?

HTH

 

[Willy beamin]

You are a smart man - thank you

Just take the compliment

 

[Island1064]

In my case I have the KO2 285/60 -18 load of 118
If I apply the load table I end up with 45PSI, am I correct?

If this is true, darn that will make the LC200 a little too stiff. Would 40 be safe?

OK, but buckle up folks, it's going to be a bumpy ride ...



It makes no difference whether the LT285/70-17 tire is D-Rated or E-Rated, the required pressure is the same.

**********
What we know now is:
For a stock LC200 running LT285/70-17 tires, the required Cold Tire Pressure is 40psi Front/Rear.
**********

See? Easy, right?

HTH

 

[Billfish]

I find that fully loaded with trip cargo my Nitto Trails run best at just below 40 psi.

At 44 psi and at roughly sea level the ride seems to be a little hash and steering apt to be a little less definite for a moment after a significant bump or correction (a little like neg caster might act).

I try to follow the 4psi increase rule from cold to hot. Its the only way to correct for load, elevation, outside temps and road surface conditions and tire build. But for me an average pressure of approx 38-39 works best. My road set of Nitto Terra Grapplers (slightly lighter build) are much the same at 40 psi average.


At 50 psi you might get more tire life but it must be like riding on marbles.

 

[gaijin]

In my case I have the KO2 285/60 -18 load of 118
If I apply the load table I end up with 45PSI, am I correct?

If this is true, darn that will make the LC200 a little too stiff. Would 40 be safe?

If you mean the LT285/60R18 118/115S D-Rated tires on a stock LC200, then you are close to correct. In order to achieve the requisite Load Limit of >2,286 lbs, you would need a Cold Tire Pressure of 46psi.

Inflating to only 40psi would yield a Load Limit of only 2,070 lbs - almost 10% less than required. This would be like running the stock P285/60-18 tires at 27psi when they should be at 33psi. Safe? I wouldn't do it.

HTH

 

[Island1064]

Thanks a lot gaijin, I will put more pressure. Guess I'm lucky, just came back from a road trip at 40 psi.

 

[JBHorne]

At 50 psi you might get more tire life but it must be like riding on marbles.

It's actually surprisingly smooth! Guess that's the awesome shocks

I'm going to air down to 45 PSI and see if I like it better.

 

[JBHorne]

Thanks a ton @gaijin for the great info and superb breakdown!

 

[gaijin]

It's actually surprisingly smooth! Guess that's the awesome shocks

I'm going to air down to 45 PSI and see if I like it better.

With your LT285/75R17 tires, 45psi will give you a Load Limit of 2,640 lbs/tire - that's 10,560 lbs Total -or- 1,300 lbs more than stock. Sounds about right for your application.

The 50psi you were running gave you a Load Limit of 2,835 lbs/tire - that's 11,340 lbs Total -or- 2,080 lbs more than stock.

HTH

 

[gaijin]

Thanks a ton @gaijin for the great info and superb breakdown!

Always glad to give some back when I can

 

[JBHorne]

With your LT285/75R17 tires, 45psi will give you a Load Limit of 2,640 lbs/tire - that's 10,560 lbs Total -or- 1,300 lbs more than stock. Sounds about right for your application.

The 50psi you were running gave you a Load Limit of 2,835 lbs/tire - that's 11,340 lbs Total -or- 2,080 lbs more than stock.

HTH

1300 lbs seems like a lot. I guess I should just stop by a weigh station and get an accurate measurement (like at the dump or something).

My own thoughts (read: random ass guessing):

Front bumper w/winch: 200
Rear bumper: 100
Sliders: 150
Skid plates: 150
Additional shock weight: 100


That's ~700, so maybe 40 psi is better? Or am I WAY off on the weight estimates?

 

[gaijin]

1300 lbs seems like a lot. I guess I should just stop by a weigh station and get an accurate measurement (like at the dump or something).

My own thoughts (read: random ass guessing):

Front bumper w/winch: 200
Rear bumper: 100
Sliders: 150
Skid plates: 150
Additional shock weight: 100


That's ~700, so maybe 40 psi is better? Or am I WAY off on the weight estimates?

Let's baseline it for you and you can go from there based on what you think the total added weight of all your accessories really is:

- If your truck were bone stock, the correct Cold Tire Pressure for your LT285/75R17 tires should be 37psi F/R which would yield a Load Limit of 2,301 lbs/tire -or- 9,204 lbs Total.

37psi = 2,301 lbs/tire -or- 9,204 lbs Total = Stock
40psi = 2,430 lbs/tire -or- 9,720 lbs Total = 516 lbs more than stock
45psi = 2,640 lbs/tire -or- 10,560 lbs Total = 1,356 lbs more than stock

Figure about 168 lbs/psi increase in Load Limit between 40psi and 45psi.

If your guesstimates are correct, your total added weight is 700 lbs over stock. This puts you between 40psi and 45psi.

Since you need about 184 lbs more Load Limit than you would have at 40psi (700 lbs - 516 lbs = 184), and you would gain 168 lbs/psi in this range, then 184 lbs / 168 lbs/psi = 1.1psi additional over 40psi = 41psi (approximately) for 700 lbs added weight over stock.

If you think you're above or below the 700 lbs guesstimate for additional weight, you can adjust up or down accordingly.

HTH

 

[Willy beamin]

Now you have me second guessing my setting - so for a 35x12.5x17 with around the same weight (700lbs) as JB?

 

[gaijin]

Now you have me second guessing my setting - so for a 35x12.5x17 with around the same weight (700lbs) as JB?

Sure! The good news is, it gets easier as we build our common knowledge base

If your truck were stock, the required Cold Tire Inflation Pressure for your 35x12.50R17LT tires would be 32.6psi

32.6psi = 2,285 lbs/tire -or- 9,140 lbs Total = Stock
35psi = 2405 lbs/tire -or- 9,620 lbs Total = 480 lbs over stock
40psi = 2625 lbs/tire -or- 10,500 lbs Total = 1,360 lbs over stock
45psi = 2840 lbs/tire -or- 11,360 lbs Total = 2,220 lbs over stock

Your additional 700 lbs over stock would put you between 35psi and 40psi.

In this pressure range, there is a Load Limit difference of approximately 176 lbs/psi.

700 lbs (your additional weight) - 480 lbs (additional Load Limit @ 35psi) = 220 lbs (required additional Load Limit above 35psi Load Limit)

220 lbs (required Load Limit increase) / 176 lbs/psi = 1.25psi additional required pressure above 35psi = 36.25psi

Round off and call it 36psi for your 35x12.50R17LT tires with 700 lbs additional weight over stock.

Looks like the 33psi you were running was perfect for a stock truck, but probably a little low for your current configuration. The 44psi you switched to was definitely too high.

HTH

 

[jmw101]

Thanks for the insight, Gaijin. Always excellent information from you.

 

[Willy beamin]

thank you very much! - I'll drop it down and test it out

 

[Island1064]

Gaijin you are the Tire Whisperer
You should build up an app.