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Proper Inflation on Over-sized tires

DieselSlug

DieselSlug

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Hey guys,

Looking for some input here as to what PSI i should run in my new Kumho KL-78's on my F250.

Currently I'm pissing off the TPMS (They call for 65 PSI) and I' currently running 50 PSI. My rims are the 20'' diameter (8'' wide) 8-spoke Lariat/King Ranch rims with 325/75/R20's on them. This size equates to a 35x12.6x20 americano.

They ride darn good where they are at, however with the salty roads here it looks as if the outer lugs don't have any "salt dust" on them like the center of the tires. This is true for the front and back. They are load range D as I rarely tow with this truck, if i do its a open car trailer with a 3K LB Fox on it.

The stock tires were 275/65/R20 which is a 34x10.8x20. (Originally had 37x12.5x20's when i bought it)
 
DieselSlug said:
Hey guys,

Looking for some input here as to what PSI i should run in my new Kumho KL-78's on my F250.

Currently I'm pissing off the TPMS (They call for 65 PSI) and I' currently running 50 PSI. My rims are the 20'' diameter (8'' wide) 8-spoke Lariat/King Ranch rims with 325/75/R20's on them. This size equates to a 35x12.6x20 americano.

They ride darn good where they are at, however with the salty roads here it looks as if the outer lugs don't have any "salt dust" on them like the center of the tires. This is true for the front and back. They are load range D as I rarely tow with this truck, if i do its a open car trailer with a 3K LB Fox on it.

The stock tires were 275/65/R20 which is a 34x10.8x20. (Originally had 37x12.5x20's when i bought it)
Click to expand...
While it's D need to know what load rating the tire has. Generally, you chalk tread the tires 1 front and 1 back and drive a short distance to see the tread that is on the ground, if the tire is riding center and sides are not on tramat then lower the air pressure 5 lbs at at time re-chalk the tread and drive again and redo until the edges of the tire touch the ground however the load rating is critical D load rage is one thing but the tire should be marked with it load range in lbs. then it has to be determined whether or not the tires air pressure is too low to bear the weight of the vehicle loaded.
 
From the sounds of it, it sounds like you need a wider rim to me. A 12 inch wide tire on an 8 inch wide rim isn't going to want to run with the tire laid down flat unless you run some REALLY low tire pressure. You need to know how wmide the actual tread is when flat, and find a rim that is prefferably 1 inch narrower, but 2 inches narrower is the max IMO.
 
With my 33x12.5 R 20 load range E on 9" wheels I ran 65 up front and 50 rear for tire wear purposes in the winter but could let my fronts down to 50 and rears near 30 but that was way too soft for me. I needed a 20x10
 
THEFERMANATOR said:
From the sounds of it, it sounds like you need a wider rim to me. A 12 inch wide tire on an 8 inch wide rim isn't going to want to run with the tire laid down flat unless you run some REALLY low tire pressure. You need to know how wmide the actual tread is when flat, and find a rim that is prefferably 1 inch narrower, but 2 inches narrower is the max IMO.
Click to expand...
Rim width got past me and yes that is the big issue.
 
Call the tire manufacturer and ask them. Otherwise, you can ask the Tire and Rim Association. My money is on over-inflation with 50 psi, unless you've got a snowplow on the front with a camper in the bed.

Inflation pressure on the tires is typically maximum pressure, only if I recall correctly.

There's some sort of footprint test you can do, too, but I don't know the procedure. I've asked about it, but haven't gotten a response.
 
Yes, that is the major issue here. Its not like I can go by the manufacturer suggested because this application is "frowned upon" by the manufacturer.

I used to run 12.5'' wide tires on 7.5'' Jeep rims all the time and as long as you found the "sweet spot" for the pressure your tire wear was normal.

Its very difficult to get skinnier tires in this flavor due to the large tire size and rim diameter unless your willing to run a skinny swamper. I really should have downsized yet again and gone with 33's, but I was worried they would look dinky.

I'm running all at 50 psi now and the truck feels great, but by looking at the tire the other day it kinda leads me to believe i may have to lower a few more PSI when i was hoping to raise it to 55.
 
SnowDrift said:
Call the tire manufacturer and ask them. Otherwise, you can ask the Tire and Rim Association. My money is on over-inflation with 50 psi, unless you've got a snowplow on the front with a camper in the bed.

Inflation pressure on the tires is typically maximum pressure, only if I recall correctly.

There's some sort of footprint test you can do, too, but I don't know the procedure. I've asked about it, but haven't gotten a response.
Click to expand...

Exactly. Its the pressure that the truck can haul the maximum amount of weight on that said tire.

I figured with the weight of the truck it would squish the tires down well at 50PSI, but second guessing.
 
FellowTraveler said:
While it's D need to know what load rating the tire has. Generally, you chalk tread the tires 1 front and 1 back and drive a short distance to see the tread that is on the ground, if the tire is riding center and sides are not on tramat then lower the air pressure 5 lbs at at time re-chalk the tread and drive again and redo until the edges of the tire touch the ground however the load rating is critical D load rage is one thing but the tire should be marked with it load range in lbs. then it has to be determined whether or not the tires air pressure is too low to bear the weight of the vehicle loaded.
Click to expand...

This chalk trick does sound interesting, will have to give it a try on a dry day.
 
So i had a guy at work tell me to try "over inflating the tires to see if the tread area of the tires will be "sucked in". Im having a real hard time believing this. He said they did it in their dirt track cars. Im down to 45 PSI and its still not quite running on the full tread. Its closer for sure, but i think i still need to lose some air.
 
Ford has a tsb about factory inflation on the front. You need max psi (factory) or you may get a death wobble type situation.

On oversized tires I don't know how low the factory front axle will handle and I don't know how the lift might change its behavior
 
tanman_2006 said:
Ford has a tsb about factory inflation on the front. You need max psi (factory) or you may get a death wobble type situation.

On oversized tires I don't know how low the factory front axle will handle and I don't know how the lift might change its behavior
Click to expand...
Interesting! I'm going to keep dropping them 5 PSI at a time for now. I drive highway everyday, so DW will rear its ugly head real fast if I went too far!
 
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