Safe Driving Speed and Tire Size on M939 Series Trucks

[goldneagle]

The answer is in the Goodyear link provided above. If you click on load and inflation information, a pdf file will open and give you all the information you want on this subject, including a new maximum load capacity if you are going over the speed rating. In short, the speed rating and max weight are meant to be used in tandem with the inflation pressure range. By changing any one, the other two can be adjusted to compensate safely.

It is there, in writing, from the manufacturer of the tire.

**Disclaimer**
Just because the tire can be safely driven at that speed does not mean that it is a safe driving speed for that particular vehicle. That is a personal decision, to be made by the individual. I would suggest restraint until very familiar with the vehicle and the tires in question.

From the PDF file:

LOAD ADJUSTMENTS FOR SPEED
A) Load Adjustments for Increased Speed (Column A on Worksheet pg. L-5)
Speeds > 65 mph for 65 mph rated tires
(Tires restricted to less than 65 mph may not be adjusted using this method.)

The Tire and Rim Association permits tire load increases, often with increased inflation pressure, for both Truck-Bus tires and Light Truck
tires used on improved surfaces at reduced operating speeds. In addition, the Tire and Rim Association also permits operating a 65 mphrated
tire at higher speeds with a reduced load and increased inflation. (The Goodyear Tire & Rubber Company does not condone or
recommend operating speeds above posted limits.) Goodyear accepts these increases, and they are published in our truck tire engineering
data book. Rim and wheel manufacturers mark their products with a maximum load and inflation. This applies regardless of operating
speed. The rim/wheel manufacturer must be contacted to determine if any deviation is permitted in the marked maximum load and inflation
capacity of the rim or wheel at the operating condition in question. For further details and a worksheet covering the use of these tables,

 

[73m819]

From the PDF file:

LOAD ADJUSTMENTS FOR SPEED
A) Load Adjustments for Increased Speed (Column A on Worksheet pg. L-5)
Speeds > 65 mph for 65 mph rated tires
(Tires restricted to less than 65 mph may not be adjusted using this method.)

The Tire and Rim Association permits tire load increases, often with increased inflation pressure, for both Truck-Bus tires and Light Truck
tires used on improved surfaces at reduced operating speeds. In addition, the Tire and Rim Association also permits operating a 65 mphrated
tire at higher speeds with a reduced load and increased inflation. (The Goodyear Tire & Rubber Company does not condone or
recommend operating speeds above posted limits.) Goodyear accepts these increases, and they are published in our truck tire engineering
data book. Rim and wheel manufacturers mark their products with a maximum load and inflation. This applies regardless of operating
speed. The rim/wheel manufacturer must be contacted to determine if any deviation is permitted in the marked maximum load and inflation
capacity of the rim or wheel at the operating condition in question. For further details and a worksheet covering the use of these tables,

Please note this

 

[spicergear]

My most humblest apologies for the thread hijack...but is the best balance for 1400x20's on CTIS trucks the three nut truck on the opposite side on the bead lock bolts or has someone come up with a specific weight for a one stud weight? I searched and found mostly A3 stuff not M939 series answers. My tandems bounce on the road a bit.

 

[Csm Davis]

Spicergear try and find a big truck tire shop that can balance on the truck. Back to the subject of speed and load the adjustment chart looks good for the 177s but the 14.00s are restricted speed tires that are 55mph max that the chart shouldn't be used for. And just so nobody thinks I am a old fart that put-putts around I have two Camaros and a Grand National with full road race suspension,and a Ninja 900. I like to go fast but as safely as possible.

 

[autoshopteacher]

Just an observation, for what it is worth. I live 11 miles from the DynoJet research manufacturing shops and (see my "handle") regularly take my high school advanced auto students there to see their R&D and manufacturing facilities. I take something from my personal stable (70 GTO, 70 GTO Judge, 69 Firebird, 08 Z06, etc) and make a dyno run. I had an 03 Hemi Dodge 3/4 ton p/u with BFG All Terrains and had the ECU program altered to do, among other things raise the speed limiter to 128 mph. The BFG's were P rated, 94mph. What we observed in tire deflection as the speed reached 120 was frightening. Tires looked like water baloons. The centrifical force at the increased speed distorted the profile enough to create bouncing on the dyno. I only presume those forces would also tax larger tires and cause self destruction quickly.

 

[Katahdin]

M939s do not have a good safety record. It would be in everyone's best interest not to push the envelope.

 

[gringeltaube]

I used this calculator trying to figure out what happens with a typical 16.00R20, at higher than rated speed.
Interesting what comes out if we considered just 30 pounds (less than 10% of total tire mass) concentrated at "M"... certainly a substantial increase in internal forces! (below)


Now, let me say this: what really kills a tire is HEAT!
Not just 20mph over rated speed limit; or overload; or pressure too low; or ambient (road) temps... BUT all factors combined; causing "internal" HEAT - and in extreme cases OVERHEATING the carcass until..... ka-booom!

In the heavy truck world that happens every day, even with brand-new tires! They normally push them to the limits: especially in Summer you can't hold your hand on the tires tread, on a loaded truck that just rolled in for a short gas stop.

Luckily not the case for 99% of our "light duty" toys, though.
If I had been doing 60mph for longer than 20 miles and all my tires remain less than hand-warm then I know I'm very well within the safe range...
And when I'm loaded I don't drive that fast... and check my tires often!


G.

 

[Csm Davis]

M939s do not have a good safety record. It would be in everyone's best interest not to push the envelope.

To be fair I believe that was before the ABS mwo

 

[skinnedknuckles]

Another thing to consider is the age of the tires in Question. I have seen Tires come apart in all sorts of ways. Sometimes you have a sidewall that may support some weight other times you don't and my tire pile here at my shop is full of both. All I can say, is that if you are cruising down the road and your steer tires are questionable due to high road speed or age and you have a major failure of the tread or carcass there would be very little you can do. Just for sh;';ts and giggles look at the height of the sidewall and realize what would happen if you lost the sidewall and that corner settled down to the rim if for a brief moment how much control would you have - if you had your thumbs wrapped around the steering wheel they would be broken and dangling there unusable so you can't hold onto the wheel so no control. Sorry about being a little graphic but it can happen. A fellow mechanic at another company lost a driver and an empty quad axle dump truck this way when a 425 float steer tire had a major failure on the interstate and made the truck swerve across the center grassy median and into the path of on-coming traffic killing a driver in a car a few years back. Not a rant but the way it can and has happened
Paul in Janesville, Wi

 

[jasonjc]

Here what a "BLOW OUT" on a 12.50R20 likes like. This is off my M35. It was on the front axle and I was going around 50-55mph. That is my size 12 foot in the hole. Don't tell me that a blow out is no big deal and you can't even tell it happened.

The other pic's are of my trailer tires 3 out of 4 blow out with the trailer sitting in the yard in the shade, within 3-4 weeks of each other. Tires have maybe 1,500 miles on them.

 

[62m52]

I'm a new member here but I was looking into putting larger tires on my m52 with the gas motor for some highway driving. Main reason was lower rpms and second was speed. The more I've looked into it, I feel that semi tires (22.5) would be a lot better and a much safer option. The tires are made for highway speeds, and they do have some brands with a little bite for off-road use. straight tread ain't getting you far! Military tires were made to go off-road at low speeds with minimal problems such as punctures. They were not designed to go down mainly highway! I would recomend getting a set of older super single semi rims (22.5)(newer style has smaller centers), get some new semi tires with some off-road type tread, and you will be a lot safer traveling at higher speeds on road. And yes.. they make large semi tires comparable to the 16.00x20. Still need to respect the 5 ton for the machine it is and don't push the envelope. Know your limitations