M38A1CDN2 Tires
 

Hi Guys
I just bought 4ea Coker/Firestone 7.00-16 NDCC tires and tubes for my 1967 M38A1CDN2 jeep, at my local tire dealer. The old ones on my jeep were 17 years old, and yes I listened to the Jay Leno story.
The question that I have is this: the jeep manual states that the highway tire pressure is 28psi. But the tire dealer says that Coker has the tire pressure stamped on the tires as 45psi.
So which is correct psi for the jeep... the Army Manual 28psi or Coker's 45psi??
Just want to figure out this big difference in psi values.

Richard Mix Jr.
1967 M38A1CDN2
CFR 6707832

I would think that the 45psi is the MAXIMUM safe operating pressure. I have never seen vehicle specific pressures on a tire but only max safe operating and max safe bead seating pressure (on tubeless tires). These are not modern commercial vehicle tires designed to run at high speeds carrying huge loads but Jeep tires which are not much more than car tires with a deep tread !

David

I don't know about Jeep tires in specific but in general the pressure on a tire's sidewall is the maximum permissable pressure.

In general the maximum weight capacity of a tire varies with load, inflation pressure and speed (and required tire life) to keep carcass temperature within acceptable/survivable limits so ifthe tires have more weight capacity at maximum inflation pressure than needed for the vehicle, inflation pressure can be safely reduced (to a pressure determined by the manufacturer) to a lesser value. According to Coker's web site max weight capacity of their Firestone 700-16 NDCC tires is 1800 pounds at the 45psi inflation. According to the M38A1 CDN2 Operator's Manual, I see maximum permissable gross weight of the winch equipped version as 4080 pounds on highway (less for off-road or without winch).
From the web, here's a link to a set of Firestone load inflation charts https://commercial.firestone.com/con...s_web_2014.pdf Unfortunately, they don't list bias ply 700-16 tires in the charts but the chart should give an idea of the relationship.



Assuming even weight distribution at the four corners of the Jeep, I see the charts as suggesting that the 28psi value may be OK. In the end, it is your Jeep, your tires, your life and your judgement/decision that matter.

What is the Jay Leno story?

45 is the maximum, and that would be around the pressure you might have to go to to seat the beads when you install them. But the 28 psi is the reccomended pressure for the M38A1, based on the gross weight. As Grant has mentioned, it will have to do with temperatures, but is also based on the amount of sidewall deflection (which is related of course). It also lets you get the most mileage out of the tread, although in your case it will be more likely that you will exceed the expiry date of the rubber before you wear out the treads on your truck.



Note that 28 psi is the maximum, so would be the jeep with a full tank, crew and payload to max weight. You could go a little lower for better traction, but really, like most of us I'll guess you are just cruising maybe a couple hundred miles a year during the spring/summer.



If you were to drive them around with 45 psi, I think it will be like driving with Fred Flinstone wooden wheels. The pneumatic tires add some (if not most) of the smoothness to the Jeep's ride.



I just set my tire pressures yesterday, all to 28 psi. It allows me to engage and disengage the 4 wheel drive without driveline binding.

This suggests similar rolling radius for both axles as a result of similar axle loads - or at least not horribly different. (so similar safety margins at both ends)

One would think it would, but there is a fairly significant difference between the front and rear axles when the vehicle is loaded. But really, aside from the driver, the passenger, and perhaps a radio set in the back, what kind of weight do we really carry in our jeeps?

Wow!
 

“Jay Leno needs about 1200 tires” :wacko:

Thanks for all of the feedback received. The tire dealer insisted that l should go by the psi listed on the tire, I did not agree with him. The tires are now set at 28 psi. Shows how much some of these people selling and installing tires really know.
I wonder what psi Jay Leno had put in his 1200 tires??

Richard Mix

I think the tire dealer was doing "cover your ass" - tires at maximum pressure rarely fail, underinflated tires have significant failures...

45 is too high, 10 would be toooo low. 28 is just right.

I cannot imagine driving an M38a1 with 45 psi. Give it a try and you'll see what I mean.

I have also noticed tires cracking prematurely when they are at max pressure. Especially the STA brand.

Hi,
I pulled out the old tires paperwork. I bought the old tires back in April 2006, and they cost $95 ea and the tubes were free. During those 17 years l put about 9000 miles on them. I had no problems or flats during their life on the jeep.
What is the average life span for some of the other jeep tires that you guys have?

Richard Mix Jr.

I put my new tires on a couple years after you did. My trips are generally short. My tires are barely broken in. They probably do as many miles offroad as highway. We use it to go to the local forest for firewood pulling a 1.5 ton trailer.