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What is proper PSI for 33" tires?

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#22 ·
Be wary of using the chalk test as your sole determiner of the correct air pressure. More often than not, it results in the tire being underinflated as the clueless attempt to get every inch of the chalk to wear off evenly.

And yes, we are talking every day chalk like kids and teachers use. :)
 
#24 ·
Kevin925 said:
How come my 33/12.50/15 say 35 psi on the tire? That's what I have had mine set at.
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The tire pressure molded onto the tire's sidewall is only its maximum safe air pressure to be used only when the tire is carrying 100% of its maximum allowable load. 35 psi would never be correct for that tire when it is only carrying the weight of a Wrangler.

Our tires NEVER carry the max safe load since that would give a zero safety margin. Typically our tires never carry more than maybe 50-60% of their max safe load.

Your tires will ride better and wear more evenly if you air them down to a lower air pressure that is more appropriate to the weight of a Wrangler. 26-28 psi max.

Think about this... the same size and model tires tires will be installed on all kinds of different types of vehicles with widely varying weights. So there is no single air pressure that can possibly be correct for them all. So the pressure molded onto the sidewall could only be correct when the tire is carrying its maximum safe permissible weight... and a Wrangler is nowhere near that so you should never air them up to 35 psi. What's terrible is that not even many tire store employees know that they're not supposed to air tires up to the sidewall pressure. They are not well educated about such things. :)
 
#27 ·
I think 24 to 26 psi would be ideal. Maybe 27 or 28 at most. IMO. That's based on my assumption that you have 8" to 8.5" wide wheels and guessing your approx Jeep weight.

Chalk test and do driving tests (a few days driving at each pressure tested) to test everyone's opinions and to develop your own opinion based on your Jeep, wheels, and tires.

I didn't see where your wheel width is mentioned. Wheel width makes a difference. I guessed/assumed you have an 8" to 8.5" wide wheel.
 
#29 ·
Kevin925 said:
They are 10" (wide) rims. Maybe that's why 31 psi feels better? What's a chalk test?
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Ah so. Well, I don't know what is best psi for a 10" wide wheel with that tire size, but my best guess is approximately 5 psi more than I recommended in my previous post when I thought you had an 8" wide wheel.

FYI - most of the posters in this thread were (IMO) assuming you have an 8" wide wheel, and were (probably) making psi suggestions based on that.

Now that I know you have a 10" wide wheel, I'm going to revise my estimated best psi up 5 psi.

So probably from 29 psi to 33 psi is likely best. Chalk test in a parking lot somewhere to see how your contact patch is at each pressure. Then drive for at least 3 days at each psi to test how it drives and handles at each, and to test ride quality and traction at each.

I like to go for the lowest psi that gives good contact patch with decent handling/cornering because I need a soft ride quality (bad back) and more traction (lots of wet pavement here year round, and snow/ice in winter). Also, it's a convenience for me because running the lowest psi that's acceptable on street is low enough to drive on bumpy rocked logging roads and for mild wheeling (like my cousin's long bumpy slick driveway). I don't want to have to air down just to drive up my cousin's driveway once a week. He follows the same policy with his truck tires for the same reasons.

If it was me, I'd probably run about 29 to 30 psi for your wheels and tires, but if you have a good spine (and can handle harder bumps) you might be happier running 33 or 34 psi and getting better gas mileage. Running 31 or 32 psi is probably a nice compromise, which might explain why your IRL experiences are telling you that 31 psi is good.

Tire wear will be better with 29 to 31 psi, IMO. So having tires last longer makes up for using a bit more gas at lower psi. Also, it's really convenient to run a psi on street that is low enough for mild wheeling on rough logging roads and long rough rural driveways. That way you don't have to stop and air down for a bumpy driveway or logging road, and you don't have to air down for mild winter conditions or rainy conditions. And you don't have to air up for highway either, if you can find a nice compromise/medium psi.

That's all I can tell you. Now it's up to you to try various psi from 28 to 35 psi, chalk test them all, and drive at each psi for at least 3 days. It'd be best to drive at each psi for one week. That way you will find our for yourself what's best for you, your Jeep weight, your suspension, your tires, your wheels, your driving conditions and style, and your climate.
 
#30 ·
Kevin925 said:
They are 10" rims. Maybe that's why 31 psi feels better? What's a chalk test?
Click to expand...
I don't have time to explain a chalk test because I've got to get off line and go somewhere.

There are many other people here who can explain a chalk test.

For now though, 31 psi is close enough for now, for your tire and wheel size and estimated Jeep weight. After full testing, you might find that 31 psi is your ideal pressure (with 10" wide wheels).
 
#32 ·
Kevin925 said:
I'll do a google search for the chalk test. 31 feels pretty good. thanks for the advice
Click to expand...
You could do a forum search for "chalk test". It's been explained many times at this forum. I've personally explained it a dozen or more times in the past.

I just can't bring myself to type that same info again. Use the forum's search feature for topics already covered previously by other threads.
 
#34 ·
Kevin925 said:
No problem. I was able to do a Google search on it. Seems like a fairly simple procedure. Thanks for your reply.
Click to expand...
P.S. - I didn't mean to sound snippy. Rereading my prior post 12 hours later I now realize it could have seemed snippy. Sorry if it seemed that way.

=====

I'm betting that you'll decide that 30 or 31 is best, after your chalk testing and more road testing. I'm curious to hear how accurate my guess is. Please let us know what you decide after your complete your tests.

I haven't used the formula to make that estimate. It's just my seat of the pants gut feeling.

=====

FYI, there is an algebra formula that very accurately helps you determine best tire pressure within a pound (plus or minus) and is more convenient than chalk test. The formula is accurate when the tire is 2" to 3" wider than rim, which is exactly your situation since your tire is 2.5" wider than your rim. I've explained the formula in past threads. The chalk test works well too, but is more work.

Also, FYI - most guys can't reliably use the formula because they have a tire that is 4.5" wider than their rim. i.e. - they have a 12.5" wide tire on an 8" wide rim. That throws the formula off by about 5 psi. However, if they use the formula and then subtract 5 psi, then it's reasonably accurate (within 2 psi, IMO)

=====

Tire stores used to use the formula, or use charts based on the formula, to recommend tire psi for a specific size and load rating tire on a specific weight vehicle. They stopped using this formula, and stopped using charts based on the formula, because of tire store fear of lawsuits (in case an employee got it wrong). So now they either use psi printed on door (which is ONLY correct for stock tire size and load rating), or they use the max psi printed on side of tire, which is intended ONLY for max load on tires.

So tire stores' fear of lawsuits has caused tire stores to be afraid to properly train their employees to use proper tire pressures. Instead they pass the buck (legally) to the car manufacturer by using the psi printed on car door, or pass the buck (legally) to the tire manufacturer by using the psi printed on tire. Either results in a WRONG tire pressure (way to high) for SUVs that have larger than stock tires. The tire stores KNOW it's the wrong psi, but they don't care because if a tire fails due to psi and a lawsuit happens, it'll be the car maker or tire maker who gets sued since it was their stated psi used.

This legal bull poop is a sign of our horse poop modern society.

Back in the day, all a guy had to do was ask a tire store and they'd either use the formula, or pull our a chart based on the formula, and they'd tell you the recommended psi for that tire on your SUV and it'd be very close or same as whatever you're going to determine from all your testing.

I'm just barely old enough to remember when most tire store employees used to know WTF they were doing. Now only the senior people at tires stores know WTF they are doing, but their bosses won't allow them to determine the best psi for your specific tire on your specific vehicle. That's because their bosses were told by their corporate lawyers to NOT allow tire store senior employees to use their knowledge in that area, and were told NOT to train the younger tire store employees how to determine proper pressure.

So then we SUV owners have to figure it out ourselves. There are many things I'm not an expert at, but I'm an old hand at this.

=====

I'm seriously considering making an Excel spreadsheet that does all the math for you, if you enter your tire's load rating, your vehicle weight, tire width and wheel width. It'd tell you recommended psi, which would be close enough to just use and trust, but if you wanted to fine tune it, you could use it as a starting point and then chalk test it and road test it, and repeat in 1 psi increments for a few psi above and below the formula's recommended psi.

IME the formula is correct within 1 psi of what the chalk test combined with driving tests will tell you, when the tire is within 2" to 3" wider than wheel. It's still correst withing 2 psi when tire is 4.5" wider than wheel, but only after subtracting 5 psi from the formula's answer.

Would anyone be interested in me making such a spreadsheet for calculating tires psi?
 
#36 ·
Really depends IMO...

My 33x14.00 Boggers were 10psi street, 3psi trail..

My 32x11.50 BFG's are 38psi on an 8" wide wheel (street) 10psi Trail..

my brothers 1ton dodge has 35" bfg Muds and i keep it at about 45psi..
 
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#37 ·
I owned an off road shop for 30 years and was part of the jeepers Jamboree comity for 18 years. 33x12.50 tires on 15" rims - light vehicles like jeeps and even smaller Cherokees - 20lbs front and 18lbs rear. Off road- 7 lbs front and 5 lbs rear. These tires don't even start really working until they are 8 lbs or less. If you have double bead locks you can drop them a little. Heavier vehicles add about 5 lbs. At 30lbs on the highway they ride like rocks and wear the center of the tires out faster. I am retired now and have a '92 Cherokee (lots of dog room) with a 406 Ford stroker and run 33's - actually General Grabbers as I drive on the highway a lot and want a smooth quiet ride I live on the Oregon coast and the dogs love the sand. This is not a dunes climber but it works well and has very good road manors and I tow it behind my motor home.
 
#39 ·
Just got new Falken Wildpeak AT3Ws for my 2020 JL in 275/70/17. They're load range C, not P metric so the tire dealer filled them up to 47psi - max on the sidewall is 50psi. They rode like solid wheels. I lowered the pressure to 44psi and they're much better. I'm going to see how it goes at that pressure and may lower to 42psi and try that.
 
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