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canyanman 05-22-2013 08:45 AM

tire pressure
 
I just got 245/75/16 Duratrac load range c on yesterday, what's the best pressure for them? The shop put 40 psi in but, the ride is a little stiff. Just wondering what you guys are running for you DD. Thanks

jserna 05-22-2013 09:05 AM

33-35.

derekmac 05-22-2013 09:27 AM

You can do the chalk test, or the math equation below, just change it for you application (below is for mine). For my 245/70-17's, it works out to be just a hair under 30psi. I've been running 32psi in them though.

Weight of Jeep: 4381 (weight is from scale)
Max psi on tire: 80

4381 x 80 = 350480

Max load on tire: 3000

3000 x 4 = 12000

350480 / 12000 = 29.2

lazyWJ 05-22-2013 09:30 AM

A good way to find the ballpark is calculate it something like this:
weight of vehicle/4. Divide that number by the load rating of the tire, and multiply it by the max tire pressure.
I add in a load safety factor as well.
For example.... 4500lbs/4= 1125 x1.5 (load factor) =1687/ 2800lbs (max tire load) =.6 x 44psi max pressure= 26. All numbers are just made up to give you the idea. Ideally you'd know your weight distribution but that's getting particular. I find the calculated results are usually a little low for my liking but they make a good starting point and I generally add air until the tire feels responsive without getting too harsh. Chalking isn't a very effective way to find optimum pressure on radial tires.

ezflip 05-22-2013 09:30 AM

Load range c tires are going to be right around 28-29 psi depending on how much cargo you normally have with you.

I run about 38 in mine right now just because Im poor and it saves a small amount on gas mileage, but optimal for ride comfort and tire wear will be just under 30.

lazyWJ 05-22-2013 09:31 AM

Derekmac beat me to the post button by 3 minutes!

derekmac 05-22-2013 09:42 AM

Quote:

Originally Posted by lazyWJ (Post 15465886)
Derekmac beat me to the post button by 3 minutes!

Our math is a little different as yours gave me 43ish psi. I think yours might be more accurate. :thumbsup: The shop had 40psi in mine when I had them mounted, but it rode a little too rough for me.

Hey, just noticed you're from Halifax, not too far away from me!

WJ_allday04 05-22-2013 09:51 AM

Quote:

Originally Posted by derekmac (Post 15465861)
You can do the chalk test, or the math equation below, just change it for you application (below is for mine). For my 245/70-17's, it works out to be just a hair under 30psi. I've been running 32psi in them though.

Weight of Jeep: 4381 (weight is from scale)
Max psi on tire: 80

4381 x 80 = 350480

Max load on tire: 3000

3000 x 4 = 12000

350480 / 12000 = 29.2

I used this formula and came up with 28.5psi for my 235/85/26 E load tires.
Is this correct/safe to drive with??? It just seems like such a difference between max psi at 80 for the tires and 29 which is what i got from the formula. Keep in mind, i used the 4in lift weight w/ 32's, provided by IRO's website. :confused:

4340lbs x 80 = 347200

3042 x 4 = 12168

347200 / 12168 = 28.5xxxxx

lazyWJ 05-22-2013 09:58 AM

Quote:

Originally Posted by derekmac (Post 15465928)
Our math is a little different as yours gave me 43ish psi. I think yours might be more accurate. :thumbsup: The shop had 40psi in mine when I had them mounted, but it rode a little too rough for me.

Hey, just noticed you're from Halifax, not too far away from me!

I didn't catch that, but yeah we're just a stone's throw! I've done the calcs with and without the "load factor" and I've found that adding the 150% seems to put me closer to the sweet spot. I don't know where that number comes from, or if it's just entirely arbitrary but it seems to get you pretty close. The butt dyno tells the tale. Once it starts getting rough you're probably at the max. That's what I do, add til it gets harsh and then back off just a hair.
I'll watch for you out there!

Quote:

Originally Posted by WJ_allday04 (Post 15465975)
I used this formula and came up with 28.5psi for my 235/85/26 E load tires.
Is this correct/safe to drive with??? It just seems like such a difference between max psi at 80 for the tires and 29 which is what i got from the formula. Keep in mind, i used the 4in lift weight w/ 32's, provided by IRO's website. :confused:

4340lbs x 80 = 347200

3042 x 4 = 12168

347200 / 12168 = 28.5xxxxx

I tried 30 in my E load 225/75/16 BFG T/A's (80psi also) and found it too soft, I settled on 38-40 which gave me the best ride/handling on the road. The math really is just a starting point to get you a minimum safe pressure.

WJ_allday04 05-22-2013 10:27 AM

Alright, thats what i thought but i wanted to be sure...thanks. My dad's a stickler about having tires pumped up correctly and i cant wait to hear his comments when i pull in at 40psi hahaha. Does running them that low affect your mpg at all?

lazyWJ 05-22-2013 11:18 AM

You won't get any significant fuel economy increases (if any at all) going from 40-80. Every PSI inflation beyond a certain point will net you diminishing decreases in rolling resistance. Depending on whose data you believe, a whopping 20% decrease in rolling resistance (difference between say, almost aired down to proper inflation) could net you something less than a 4% increase in actual fuel economy.

WJ_allday04 05-22-2013 11:24 AM

I worded that question wrong, i was meaning a loss, not gain, my bad. But your point makes sense.

alfaitalia 05-22-2013 03:50 PM

I have been experimenting with tire pressure versus mpg. My tires are extra load General grabber ATs 235-65/17 max pressure 50 psI. I started at 33 psI and have been going up 1 psI every fill up. 33 was too soft and she handled like a pig on stilts as they say round here. The idea is to see at what point the mpg stops improving with pressure or whether I get to a point where the ride is too hard to continue increasing the pressure. So far I'm up to 42 and the ride is firm but fine ...handling is sharper too... and the mpg is still improving.

cde02ltd 05-22-2013 07:42 PM

Quote:

Originally Posted by alfaitalia
I have been experimenting with tire pressure versus mpg. My tires are extra load General grabber ATs 235-65/17 max pressure 50 psI. I started at 33 psI and have been going up 1 psI every fill up. 33 was too soft and she handled like a pig on stilts as they say round here. The idea is to see at what point the mpg stops improving with pressure or whether I get to a point where the ride is too hard to continue increasing the pressure. So far I'm up to 42 and the ride is firm but fine ...handling is sharper too... and the mpg is still improving.

Doesn't the tire pressure system start screaming at you? If I put 40 or more in mine, once you get on the highway the. Pressure increases, and the evic goes nuts when the tires hit 44psi. Have you found a way to adjust when it goes off?

lazyWJ 05-22-2013 08:28 PM

If you could inflate the tire to the point that it didn't touch the ground at all, you could generally expect to see less than 10% increase in your fuel economy. In other words, if you're getting 15mpg now with properly inflated tires, filling them with flubber so they float above the pavement could net you about a 1.5mpg increase. Adding any amount of air beyond proper inflation will land you somewhere in between those numbers. Something to keep in mind for perspective.


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