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Proper Tire Inflation for Aftermarket Tires

John Bronco

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Since this came up yesterday, I thought it would be a good thing to share.

Most people assume the tire shop knows (or cares) what PSI you should run in your aftermarket tires.

Most shops fill according to the door sticker or close to the max of the tire.

For example, I just got BFG KM3s on my Ram 2500 yesterday. The off road shop who installed them is legit. Owner races Trophy Trucks and they do a whole lot of work in the area.

They filled each corner to 65psi.
Each tire is rated at 3,970lbs @ 75psi.
At 65psi, this equates to 13,763lbs capacity. (65psi is 87% of the 75psi max. 87% x 3,970lbs = 3,441lbs. 3,440.67lbs X 4 = 13,762.67)
GVWR is 9,000 for my truck.
At 65psi, I'd riding around with enough pressure in my tires to support 4,762.67 additional pounds. No Bueno.

The Door sticker has 50 psi front and 70 psi rear.
These tires inflated per the door sticker support 12,704lbs (3,704lbs excess).

70psi in the rear it is a a miserable ride. When I tow a trailer (especially my gooseneck), I'll air up for sure but in a pickup with no load in the back, you're generally going to run less in the rear than in the front. (the distribution is more front biased in a diesel due to engine weight)

At 45psi front and 40psi rear, I'd be running at 8,998.67lbs (almost exactly GVWR).
At 50psi front and 45psi rear, I am running 10,057 about 1,057lbs over GVWR. Ride is good, steering is tighter, provides a margin of error for overloading and tires should wear evenly. (anywhere between these two settings should be fine)

This is a great write-up on this and goes into much more detail.
https://carlisuspension.com/tires-and-wheels-how-do-they-affect-my-ride/

* this calculation method is for on road PSI. Off road is situational. Example: I my buggy, I run 7-10 psi front and 4-6 rear on 39" Red Label Krawlers with beadlocks.

Hope this helps someone.
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88to21bronco

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Clubs
 
Since this came up yesterday, I thought it would be a good thing to share.

Most people assume the tire shop knows (or cares) what PSI you should run in your aftermarket tires.

Most shops fill according to the door sticker or close to the max of the tire.

For example, I just got BFG KM3s on my Ram 2500 yesterday. The off road shop who installed them is legit. Owner races Trophy Trucks and they do a whole lot of work in the area.

They filled each corner to 65psi.
Each tire is rated at 3,970lbs @ 75psi.
At 65psi, this equates to 13,763lbs capacity. (65psi is 87% of the 75psi max. 87% x 3,970lbs = 3,441lbs. 3,440.67lbs X 4 = 13,762.67)
GVWR is 9,000 for my truck.
At 65psi, I'd riding around with enough pressure in my tires to support 4,762.67 additional pounds. No Bueno.

The Door sticker has 50 psi front and 70 psi rear.
These tires inflated per the door sticker support 12,704lbs (3,704lbs excess).

70psi in the rear it is a a miserable ride. When I towing a trailer (especially my gooseneck), I'll air up for sure but in a pickup with no load in the back, you're generally going to run less in the rear than in the front. (the distribution is more front biased in a diesel due to engine weight)

At 45psi front and 40psi rear, I'd be running at 8,998.67lbs (almost exactly GVWR).
At 50psi front and 45psi rear, I am running running 10,057 about 1,057lbs over GVWR. Ride is good, steering is tighter, provides a margin of error for overloading and tires should wear evenly. (anywhere between these two settings should be fine)

This is a great write-up on this and goes into much more detail.
https://carlisuspension.com/tires-and-wheels-how-do-they-affect-my-ride/

* this calculation method is for on road PSI. Off road is situational. Example: I my buggy, I run 7-10 psi front and 4-6 rear on 39" Red Label Krawlers with beadlocks.

Hope this helps someone.
Very nice write up.

If people did these calcs a lot fewer would complain about the ride of load range E tires. Just because the sidewall says xxxx load at yy pressure does not mean you need to run yy pressure in YOUR situation. That said, in a fairly balanced (front/back) vehicle with a fairly low GVWR, like the Bronco, the door sticker makes a good starting point modified for tire load rating and size.

Question for you: I've noted the door sticker for the smaller (255 x17 w/ 111 LR) tires on a Bronco says 35 psi, the 315 x 17 sticker says 39psi. Intuitively, wouldn't the 315 need less pressure to handle the same, or very similar, load?
 
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John Bronco

John Bronco

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Very nice write up.

If people did these calcs a lot fewer would complain about the ride of load range E tires. Just because the sidewall says xxxx load at yy pressure does not mean you need to run yy pressure in YOUR situation. That said, in a fairly balanced (front/back) vehicle with a fairly low GVWR, like the Bronco, the door sticker makes a good starting point modified for tire load rating and size.

Question for you: I've noted the door sticker for the smaller (255 x17 w/ 111 LR) tires on a Bronco says 35 psi, the 315 x 17 sticker says 39psi. Intuitively, wouldn't the 315 need less pressure to handle the same, or very similar, load?
Really comes down to construction of the tire. Each tire is constructed differently. A Toyo m/t will be different than a Bfg a/t which is different than a Goodyear whatever.
my old tires were 35s and max inflation was @ 65psi (don’t recall weight). These are 37s and max inflation is 75psi.
Each calculation will be specific to the tire rating, the vehicle, accessories, etc.
even the different models of bronco should be different. (Base vs badlands with lux and Sasquatch)
 

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TRMFAM

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Great info...thanks!
 
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John Bronco

John Bronco

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Almost everyone runs too much air pressure.
Yep. I get it. Most people don’t even think about this stuff. Not know where to even start looking for the info.
But those people will go leave bad tire review.
 

ROKBRNC

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If you don’t have access to a vehicle scale to measure front and rear weights and an abacus you can always do this. Remember the max PSI on the tire sidewall is only related to the tire itself, proper PSI for the load and unsprung weight will vary based on many different variations

I run 40” bias-ply IROKs with steel beadlocks on my ‘04 LJ with 1-ton axles. Using this technique I’m at 26 PSI front, 23 PSI rear, ride is controlled and somewhat comfortable considering the vehicle and tread wear is consistent sidewall-to-sidewall.

 
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Silver-Bolt

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I do a chalk test. Inflate the tires to the MFGr spec's. Make a wide chalk mark across the width of the tread. Drive a mile or two with both left and right turns. Check the chalk mark. If the mark is gone in the center but not on the edges, over inflated. If the chalk is gone from the edges but remains in the center, under inflated. Adjust the air and repeat until the chalk is evenly removed.
 

HotdogThud

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If you don’t have access to a vehicle scale to measure front and rear weights and an abacus you can always do this. Remember the max PSI on the tire sidewall is only related to the tire itself, proper PSI for the load and unsprung weight will vary based on many different variations

I run 40” bias-ply IROKs with steel beadlocks on my ‘04 LJ with 1-ton axles. Using this technique I’m at 26 PSI front, 23 PSI rear, ride is controlled and somewhat comfortable considering the vehicle and tread wear is consistent sidewall-to-sidewall.

I thought this was yet another youtuber going for likes, comments, and subscribes, until I saw him mention "And when you lift your vehicle, or go with bigger tires, or whatever. make sure you also adjust your headlights, for oncoming drivers, and safety."

if I had a nickel for every driver in a lifted brodozer with their headlights pointing directly at my mirrors instead of the road, I could pay ADM on a FE and have enough leftover to buy a spare.
 

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NonfatWombat

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This is great info and not always considered by owners. The 35" Duratracs I had on my Wrangler were air-down to 32 psi on highway - a world of difference, both in wear and handling.
 

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It’s been my experience that unless you are a 3/4 to or bigger, go with what’s on the door for daily driving.
example: my wife had a BMW Z3 years many years ago. Put new tires on a discount tire and, with all my vehicle, rotate and balance every 5k miles. Discount tire did all the balancing and rotations and at 15k miles she needed new tires. I looked at all her receipts (discount tire doesn’t charge but gives a receipt) and they had been putting 33psi in the tires on the front. Door called for 29psi and that’s why center wore out prematurely. Discount Tire replaced the tires at no charge when I brought it to their attention.
 

BigTex

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I’ve had BFG’s on many many vehicles prior to Nitto Trail Grapple release and I can say for sure that I went by the pressure on the door on all but the Ford Excursion ( I ran 50 front/45 back on it). I have never got less than 60k miles on BFG’s and over 80k miles a couple of times
 

kodiakisland

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A good place to start is the same load rating equivalent pressure, regardless of whether it is Pmetric, C, D, or E tire.

I don't have a Bronco tire sticker, so using my Tacoma as an example, the door jam sticker is for P265/75/16 @ 30PSI. That is around 2200lb load. Lets say I replace them with LT265/70/17 C range tires, it would be around 42PSI, or say a LT285/70/18 E tire, it would be around 38PSI. Whatever tire I use, I need to start around whatever PSI supports 2200lbs.

Use a chart like this to find your weight rating: How to apply the load inflation tables (toyotires.com)

Fine tune your pressure for your vehicle as you use it. The tire placard in the door jam is for GVWR. If you don't run anywhere near that, you can use less pressure. If you exceed it, use more.

I expect the E rated tires I am putting on my Bronco to be around 34-36PSI for daily use.
 
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John Bronco

John Bronco

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I understand that you guys feel that the sticker is sufficient but unless you replace the stock tires on you vehicle with the same tire or a tire with very similar load numbers (weight @ psi) the sticker is no longer the go to.

Had you done the math, chalk test or both, you may have found those tires would have rode a lot better than they did. Of course there is a window of acceptable inflation.

For the most part, stock tires and aftermarket A/T and M/T tires have very different characteristics.

@kodiakisland curious why you’d want E rated tires for a bronco. (No judgement. Just curious)
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