Tire speed help

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theboom

New member
I’ve got a new Ford f550 that I plan on using to haul a large truck camper (11-12’ floor) and I think I’ve decided I want to do a super single conversion. The biggest concern I have is the speed rating on the tires. This truck will spend a lot of time with the camper off and while I will never go over ~65 with the camper on, with it off I would really like to be able to go faster.

The tire I would like to go with is the continental mpt 81 335/80r20 for several reasons. Now the rating on them is 68mph and a little over 6700lbs per tire. With the camper off the truck weighs about 11,500lbs with about 6000lbs on the front axle and 5500lbs on the rear. I have always heard that a tire speed rating is with the tire at its max weight capacity. With the front tires only having 3000lbs on each which is well less than half, what do you think is the fastest I could safely go? I would really like to be able to do 80 without worrying too much. Another point to add is that I don’t let my tires get super low on tread and I frequently rotate them and constantly monitor the psi in them.
 
IdaSHO

IdaSHO

IDACAMPER
Shouldnt be an issue, but Id advise monitoring tire temps.

You should also contact the MFG directly, see what they have to say.
 
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charlieaarons

Explorer
Exceeding speed ratings risks disaster.
Goodyear G275 MSA 335/80R20 is rated at 81mph. For safety run them at least 5 psi over pressure dictated by load if running right up at the rating. The rating is there for a reason.
 
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CampStewart

Observer
What are the specs on this F550 that it has 6000lbs over the front axle when empty? How much wqould it have over the front axle with the camper on? What is the front axle weight rating?
 
Ravenmad

Ravenmad

Observer
I have been running Conti. MPT81s for almost a year now, around 18K on the truck and the tires since it was brand new. I agree with the load / speed rating and the effects of load in comparison to speed. In the end HEAT is the big killer of these tires, second to that is shock dampening (the overall weight of the tire / wheel combo can very easily overpower a mediocre shock). I rotated mine every 2500 miles and was very careful to get the correct tire pressure. These tires have deff. weaknesses and deff. strengths. I love them, but also realize their limitations. I have been running them as my highway tires / everyday tires and I am about to buy a dedicated set of daily driving tires. I did just replace both my front tires as they wore much faster on the front of the truck than on the rear. The tires do balance very easily and are smooth on the road, but hey are noisy (almost bogger noisy).
 
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LeftCoastKiteboarding

Active member
Meh... pressures are supposed to be set cold. This allows for about 5-10 psi increase at driving temp. If you go high to start you’re going to end up pretty high.

I agree in general with the comments but if tires are rated for 68mph, going freeway speeds of 70ish while empty is NOT going to be an issue. Going over that speed while loaded is another story.

There is a safety margin built as well. Don’t see the OP actually having issues using those shoes as DDs. It’s a big truck. Not a sports car.


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charlieaarons

Explorer
How many of the posters on this thread actually own a heavy vehicle that has 335s or larger and have run them for over 10 years with constant temperature readings?
Sorry for being catty but tires are a serious safety issue and I do know a bit about MPT tires.
Admittedly I don’t know about Earthroamer’s experiences with 335s rated at 68mph and that could be instructive. I DO know about Unimog experiences running fast with 395s.
And my thought about running 5 psi high did not mean to go over max recommended cold psi. It meant go 5 psi over psi prescribed for measured load which hopefully is not 6700 lb/tire but significantly lower. Although there is no indication by the OP of how heavy his camper will be. Hopefully less than 6000 lb which would take his RAW up to 11500, less than maximum capacity of 13400. Of course all of this is moot if he gets the Goodyears which will probably wear better than the Contis anyway.
 
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LeftCoastKiteboarding

Active member
Well, I do run 315s at max load. I was sensitive to temps for a bit until I got familiar with them. Temps are definitely the important factor. More critical than the speed on the sidewall. I’m sure the OP can run a little over max speed while empty. A check on temps would be a great check. Ymmv.


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theboom

New member
@Leftcoast - what are you referring to when you ask about running over pressure?

@CampStewart - well the trucks front axle rating is 7500lbs. About it having an empty weight of 6000lbs in the front, it’s a loaded lariat with the 6.7 and 4x4 so about 5200lbs on the front stock. Add a heavy winch front bumper with 16k winch, upgraded suspension, each wheel and tire combo adds around 100lbs each, a rear bed that’s center of gravity is in front of the rear axle and a driver, it easily gets up to 6000lbs. With the camper it’s doesn't really change because the camper center of gravity is right over the rear axle.

@Charlie - the reason I want to go with the mpt 81’s is that there is a company that sells them as a combo with a custom refurbished 3 piece aluminum double beadlock wheel and they have been legally re-rated at 7400lbs per tire with that wheel/tire combo allowing you to keep the full 14,700lb rear axle rating that it has stock.

Thanks for all the reply’s so far. Hopefully I can get a few more about driving up to 80 with the tires. I would never go over 80 but I would really like to be able to set the cruise at 80 as that is what literally everyone does where I live and I’d like to be able to go with the flow of traffic. (Also I’d be lying if I said going 70 doesn’t feel very slow to me)
 
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charlieaarons

Explorer
I checked dbldesign website. Have you found out from them if the 7400 lb rating is still at 68mph? I’ve heard of tire mfgs rating down load to uprate speed (viz. Michelin “singular point” and moving LR on 395 XZLs from 168 to 158 to allow 70mph for U500NA) but never up rating load without down rating speed. I’d like to see the documentation.
 
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quest450

New member
@theboom I wonder how did it work out for you as I'm about to get the same set of MPT 81 tires and super single conversion from DBL Designs for my 2020 F450
 
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