Safe Driving Speed and Tire Size on M939 Series Trucks

[goldneagle]

I have a M931 with Goodyear 11.00-20 on it. I got 60 MPH during the recovery trip home. (based on GPS unit)

I was going to Super Single the truck with 14.00-20's to get more speed. After checking the spec's on the tires I do not think I will follow through with change. (not an upgrade)

The Michelin 14.00-20 and Goodyear 14.00-20 both have a maximum rated speed of 55 MPH.

The Michelin and Goodyear 11.00-20's are rated 68 MPH.

I was going to Upgrade to the 14.00-20 in order to gain MPH speed. I do not think it would be safe to exceed the Maximum rated speed of the tires! So I would actually lose speed instead of gaining it! (i can do 60 with what I have) Obviously the truck could go about 12% faster (67 MPH) but exceed the maximum rated speed by about 12 MPH.

I only found one tire that could be considered an upgrade for the M931 truck. It's the Goodyear MV/T 395/85R20 tire -which is rated 68 MPH according to the spec sheet. That would give a 7% increase in speed or 4 MPH. Since 64 MPH is under the 68 MPH maximum rated speed the tire would be safe to use as an upgrade for the 11.00 tire setup.

I am only discussing tire safety on Maximum Safe Vehicle Speed.

I would love some feedback on this topic.....

 

[Squirt-Truck]

Finally someone realizes that there are posted maximum safe speeds for these tires.

You are being very wise in your question. Please also consider that these vehicles were not designed for operation at those speeds, yes you have good brakes, but the handling characteristics of the vehicle are not intended for highway use like a commercial truck.

Trust me loosing a truck tire on the prime mover is a real experience, bot in danger and damage. (Just ask 73-819 for his input...)

Just be patient with the speed you have.

 

[markmontana]

Hmmm--- so all the 1000's of M939A1'a and A2's with 1400 tires, as issued by US, are what?

 

[porkysplace]

Hmmm--- so all the 1000's of M939A1'a and A2's with 1400 tires, as issued by US, are what?

Are run at convoy speed of 45 to 50 mph.

 

[Csm Davis]

Hmmm--- so all the 1000's of M939A1'a and A2's with 1400 tires, as issued by US, are what?

Not to be driven over 55mph.

 

[eldgenb]

those speed ratings on the tires are on there for maximum rated speed at capacity of the tires, most of those tires are rated at 10k lbs each plus. If you are hauling at capacity that is the maximum allowable safe speed you can obtain. These are not 200 mph monsters that are going to spin the bead off the wheel, stay under 80 and you are fine, if you are hauling, slow down. My M920 which is 32k lbs with 16.00 r 20's are more than stable at 70-75 mph, don't over drive your trucks ability no matter what tires are on there and most of all use common sense.

 

[4x4e350]

What are you basing your "max" speed on? Just because you run your engine at 2500 RPM's max, doesn't mean that is the recommended speed of the truck. You didn't mention anything about engine RPM.
I went to larger tires to lower my cruising RPM, not gain a few MPH's. I also gained some economy by doing so. But I baby the engine in my truck, and realize the limitations of the truck.

 

[73m819]

those speed ratings on the tires are on there for maximum rated speed at capacity of the tires, most of those tires are rated at 10k lbs each plus. If you are hauling at capacity that is the maximum allowable safe speed you can obtain. These are not 200 mph monsters that are going to spin the bead off the wheel, stay under 80 and you are fine, if you are hauling, slow down. My M920 which is 32k lbs with 16.00 r 20's are more than stable at 70-75 mph, don't over drive your trucks ability no matter what tires are on there and most of all use common sense.

What 1600x20 is rated at 70-75, no matter what you say about LOADED speed, If something happens due to the 1600 doing this speed, loaded or not, WILL guarantee that YOU own the event.

When you look at tire specs, it shows RATED speed, NOT load rated speed or no load rated speed.

 

[73m819]

What are you basing your "max" speed on? Just because you run your engine at 2500 RPM's max, doesn't mean that is the recommended speed of the truck. You didn't mention anything about engine RPM.
I went to larger tires to lower my cruising RPM, not gain a few MPH's. I also gained some economy by doing so. But I baby the engine in my truck, and realize the limitations of the truck.

I see no problem with this as long as the tires stay under the rated speed, a very GOOD reason for a tire/gear change,

The problem I have is those that put BIG tires on to go FASTER (above the rated speed), then complain about no power to get to and maintain that speed at MAX RPM. One of these day, one of these big tires going 65-70 is going to blow and ending up KILLING someone, now when this happens it will be a banner day for the MV hobby world.

Blowing a steer tire at rated speed, let alone going faster is NOT something I wish on ANYONE, even a GO FASTER, the ONLY reason I am here to rant or someone else did not get hurt is because GOD had his hand on my shoulder, anybody who saw the aftermath of my event will say the same.

Some DO NOT like my rants against GO FASTERS, I really DO NOT CARE, I worry more about the MV world and the people affected by DUMB choices then a go fasters likes and dislikes.

 

[goldneagle]

I see no problem with this as long as the tires stay under the rated speed, a very GOOD reason for a tire/gear change,

The problem I have is those that put BIG tires on to go FASTER (above the rated speed), then complain about no power to get to and maintain that speed at MAX RPM. One of these day, one of these big tires going 65-70 is going to blow and ending up KILLING someone, now when this happens it will be a banner day for the MV hobby world.

Some DO NOT like my rants against GO FASTERS, I really DO NOT CARE, I worry more about the MV world and the people affected by DUMB choices then a go fasters likes and dislikes.

Yes my intent was to go faster, but not by ignoring the safe operating speed of the tires chosen. That is why i brought up the subject. I do not think all members were aware of the maximum speed rating of the 14.00-20 tires. Unless I can find some of the Goodyear 395's at a good price I will stick with the 11.00-20 Goodyears.

 

[cbvet]

I like the 1400s on my M813.
I've never carried any significant load, but it rides great at 50-55 mph.

 

[Csm Davis]

those speed ratings on the tires are on there for maximum rated speed at capacity of the tires,

Can you show us that in writing from a tire manufacture? If so I would like to see it because all the speed ratings I have been told about in the past were you go over that speed and the tire sees so much stress it starts to heat up and come apart, don't take this as a shot at you I have pushed the max speed on these and other tires, I just want to go by facts that can be proven for when I push the limits of my machines.

 

[swiss]

68 mph speed rating on a G177

Goodyear Commercial Truck Tires - G177 Details

 

[flibob]

I gotta agree that every time you go over convoy speed you are rolling the dice not only for yourself but all those around you. These are huge trucks designed to cross the roadless battlefield.

As somebody said, "good common sense sure helps". Drive safe out there.

 

[goldneagle]

68 mph speed rating on a G177

Goodyear Commercial Truck Tires - G177 Details

That is what I have on my M931.

 

[swiss]

That is what I have on my M931.

Get the 14's and sell me the G177's

 

[goldneagle]

Get the 14's and sell me the G177's

I would be willing to trade my 11.00-20 for some 90% Goodyear 395/85R20 MV/T with rims.

 

[Beerslayer]

We've had this discussion before

I've had the right front blowout at 65 MPH on a 1600.20 and can say that it is ~not a terrifying experience.

First it was "What's that noise?" then, "I hear what sounds like a big air leak, sounds like a tire but I can't feel any difference" and then, "Let's pull over and check it out!" 1/4 mile later at a safe spot I pulled over and the right front was flat.

Those 1600.20 tires are so heavy that you can't hardly tell when they are flat.

I know some of you feel pretty strongly about this and I don't mind your rants at all. Rant away! That doesn't mean you are right about this.

The tire guy I trust with 45 years of servicing heavy truck tires says that those tire speed ratings are for maximum load at maximum ambient temperatures. That doesn't make them handle any better, but my M923 on 1600.20 tires handles like a dream at freeway speeds. Running empty on dry pavement I have plenty of control, braking power, and time to react given the way I drive.

So here's a photo of the tire that blew out. See how high the rim still is from the ground on a complete blowout/flat? I couldn't even tell it was flat when I pulled over. I had to walk around the truck twice to find it in the dark with a flashlight.

 

[Csm Davis]

We've had this discussion before

I've had the right front blowout at 65 MPH on a 1600.20 and can say that it is ~not a terrifying experience.

First it was "What's that noise?" then, "I hear what sounds like a big air leak, sounds like a tire but I can't feel any difference" and then, "Let's pull over and check it out!" 1/4 mile later at a safe spot I pulled over and the right front was flat.

Those 1600.20 tires are so heavy that you can't hardly tell when they are flat.

I know some of you feel pretty strongly about this and I don't mind your rants at all. Rant away! That doesn't mean you are right about this.

The tire guy I trust with 45 years of servicing heavy truck tires says that those tire speed ratings are for maximum load at maximum ambient temperatures. That doesn't make them handle any better, but my M923 on 1600.20 tires handles like a dream at freeway speeds. Running empty on dry pavement I have plenty of control, braking power, and time to react given the way I drive.

So here's a photo of the tire that blew out. See how high the rim still is from the ground on a complete blowout/flat? I couldn't even tell it was flat when I pulled over. I had to walk around the truck twice to find it in the dark with a flashlight.

That's not a blowout that's a flat. I have had 14.00 x 20s blowout and I can say that if a fully aired up 14 blows out it sounds like a hand grenade going off and will scare the crap out of most folks. And at 55+ they come apart faster than you can slow down and will beat the fenders off your trucks I have one sitting at my uncles house because of this happening to my truck that I was bringing home, it is still there a month later because I can't change a tire right now. As to your long time tire guy even if he is the store owner I will not fully believe him until he shows it to me in writing, my brother-inlaw has work for a tire company 30 plus years and I still find things about tires that he doesn't know from time to time and this may be one of those times. I am going to ask him to find out from Goodyear which it is, speed at max load and temp or just the speed and ask if they can point it out in writing or on the Web.

 

[klmartin62]

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.