Driving faster than a tires speed rating?
- Thread starter 01f350
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charlieaarons
Explorer
Gurkha said:
Using the instantaneous display of tire pressure on my CTIS display as a guide, my 395/85R20s reach steady-state temperature in about 15-20 minutes of high speed running. "High speed" in my truck means 60-62mph; the tires are rated at 70.
I believe 335/80R20 MPT81s are rated at 68 mph (read the official Continental website). Remember that rotational stresses on a tire go up as the square of the speed. I don't think 70 would be too dangerous, especially with 5-10 psi extra air (as long as you are well below max load); but 80 would be playing Russian roulette.
Charlie
Last edited:
softboyled
New member
Devil's advocate.
Not a good idea.
eepwall:
Not a good idea.
eepwall:
R_Lefebvre
Expedition Leader
Tires are designed and rated using engineering factors of safety just like any other item. As evidenced by the Firestone/Explorer tire fiasco, usually several of the design factors must be exceeded simultaneously before a problem really occurs. In that particular case, the tires which failed were generally underflated, overloaded, oversped, in hot weather, and even then only tires with manufacturing defects failed.
That being said, there are aspects to the tires other than just their speed rating to be concerned about. Namely the way they will handle. I don't know why you'd want to be driving a truck with these tires faster than 60 anyway. I generally don't go faster than 60 in my Discovery with A/T tires. Don't get me wrong, I'm no speed ninny, I've been within an inch of losing my license on several occaisions when I was daily driving my cars. But I don't like taking sluggish, high CG vehicles up to 75, and I'm shocked at how many people do.
The speed limits are set for the lowest common denominator, and WE are it.
That being said, there are aspects to the tires other than just their speed rating to be concerned about. Namely the way they will handle. I don't know why you'd want to be driving a truck with these tires faster than 60 anyway. I generally don't go faster than 60 in my Discovery with A/T tires. Don't get me wrong, I'm no speed ninny, I've been within an inch of losing my license on several occaisions when I was daily driving my cars. But I don't like taking sluggish, high CG vehicles up to 75, and I'm shocked at how many people do.
The speed limits are set for the lowest common denominator, and WE are it.
cruiser guy
Explorer
I'm almost surprised to see this on an expedition site! Usually we are nowhere near a place we can run at speeds in excess of 60 - 70 mph legally for an extended time so it becomes a moot point. I will still allow my truck to "coast" downhill and on occasion reach speeds of 70 - 80+ mph but this is only for short times and purely as a fuel saving measure.
I could see it different if we were talking about driving across Europe with the Autobahn and their unlimited speeds but in North America and the rest of the world we are limited either legally or by road quality/conditions.
I could see it different if we were talking about driving across Europe with the Autobahn and their unlimited speeds but in North America and the rest of the world we are limited either legally or by road quality/conditions.
strong_like_tractor
New member
Back when I was running XLM's on my Superduty I did some research on speed rating of tires. What I found was that the rating applies to maximum load at maximum pressure. For example 325/85R16 XML's are rated at 5017lbs@75psi and 65mph - there's no way I was even close to those ratings. I easily drove 75-80mph from Vancouver BC to Moab with no issues. If you are worried about thermal expansion when driving at higher speeds then inflate them with nitrogen - it's much more stable.
JP
JP
I am assuming from your board name that you drive an '01 F350 pickup. If not, please correct. If so, there have to be a ton of better tires out there than the ones you are looking at. For that matter, I didn't know DOT approved any tires for use on the highway with a speed rating of less than 80mph.
I would answer your question of whether you can go above the rating as a qualified yes, but since you have a vehicle capable (I assume) of cruising regularly above 68mph, and with most highways hereabouts having speed limits of 70 to 75, I would say "no" if you intend to drive routinely at or above the limit of the tire. Once in a while won't hurt, but plan to stay at or under 65 if you choose such a slow speed tire.
How fast we drive is something we get used to and take for granted. Using the GPS to track peak speed is sometimes enlightening. I stopped driving fast a long time ago. I used to think it was fun to do 105 in my old VW but those days are gone. However, driving on a 75mph highway, I will sometimes pass traffic, and have seen speeds of 100+ as a peak speed. I would have had no idea I was going that fast if the machine didn't tell me, because I don't track the speedometer while doing such maneuvers. So, I think we often go faster than we think, and if you drive the same way more or less, I would say your choice of tires won't have enough of a safety margin. If you ever carry a load in a heavy duty truck, pushing the safety margin is just not wise.
I would answer your question of whether you can go above the rating as a qualified yes, but since you have a vehicle capable (I assume) of cruising regularly above 68mph, and with most highways hereabouts having speed limits of 70 to 75, I would say "no" if you intend to drive routinely at or above the limit of the tire. Once in a while won't hurt, but plan to stay at or under 65 if you choose such a slow speed tire.
How fast we drive is something we get used to and take for granted. Using the GPS to track peak speed is sometimes enlightening. I stopped driving fast a long time ago. I used to think it was fun to do 105 in my old VW but those days are gone. However, driving on a 75mph highway, I will sometimes pass traffic, and have seen speeds of 100+ as a peak speed. I would have had no idea I was going that fast if the machine didn't tell me, because I don't track the speedometer while doing such maneuvers. So, I think we often go faster than we think, and if you drive the same way more or less, I would say your choice of tires won't have enough of a safety margin. If you ever carry a load in a heavy duty truck, pushing the safety margin is just not wise.
R_Lefebvre
Expedition Leader
The only place I am not aware of my maximum speed is on a racetrack, where there isn't time. Otherwise I'm keenly aware of my speed, if only for legal reasons. I'm surprised to hear that you would find yourself unaware of your actual speed if not for the GPS?
Again, I'm no speed nanny. I regularly keep to 100km/h in the Disco for personal safety reasons. I'm always keenly aware that this thing will take a long time to stop from that distance, and any attempts to swerve will only result in hitting the obstacle with the roof first. In my Focus, I'd cruise at 200km/h if the road was clear and it wasn't illegal.
When I was a car guy, I didn't actually realize how crazy it is to see all these SUV's barrelling down the road at 85mph. After I got my Disco and tried it myself, I think your average SUV driver is insane.
Maybe my perceptions are swayed by the fact that I'm friends with a light truck vehicle dynamics engineer who regularly drives with outriggers fitted, and regularly warns me high CG SUV's *really are* completely unstable, and that even the most skilled driver is incapable of stopping a roll once it starts.
Again, I'm no speed nanny. I regularly keep to 100km/h in the Disco for personal safety reasons. I'm always keenly aware that this thing will take a long time to stop from that distance, and any attempts to swerve will only result in hitting the obstacle with the roof first.
In my Focus, I'd cruise at 200km/h if the road was clear and it wasn't illegal.When I was a car guy, I didn't actually realize how crazy it is to see all these SUV's barrelling down the road at 85mph. After I got my Disco and tried it myself, I think your average SUV driver is insane.
Maybe my perceptions are swayed by the fact that I'm friends with a light truck vehicle dynamics engineer who regularly drives with outriggers fitted, and regularly warns me high CG SUV's *really are* completely unstable, and that even the most skilled driver is incapable of stopping a roll once it starts.
LandCruiserPhil
Expedition Leader
I find it funny that the original question is about tires and some feel the need to offer fatherly advice on how fast one should drive. Boo-hoo
I dont know where everyone is from but out west speed limits of 75 is not uncommon. It looks to me like the guy just wants to go with the flow of traffic.
I vote for making sure your tires are moisture free or run nitrogen, properly inflated and go the speed limit.
I dont know where everyone is from but out west speed limits of 75 is not uncommon. It looks to me like the guy just wants to go with the flow of traffic.
I vote for making sure your tires are moisture free or run nitrogen, properly inflated and go the speed limit.
R_Lefebvre
Expedition Leader
75 is a legal limit, not a mimimum. But I digress.
My original point of the sidetrack was just to say that the ratings provides some indication not only of it's ability to withstand speed, but also it's handling capabilities. It's just that much more dangerous to drive a truck over 60 if equiped with tractor tires.
My original point of the sidetrack was just to say that the ratings provides some indication not only of it's ability to withstand speed, but also it's handling capabilities. It's just that much more dangerous to drive a truck over 60 if equiped with tractor tires.
I don't drive an SUV. The Audi A6 2.7T Quattro will easily reach 140 if you have the time, and 110 comes up so fast you won't even notice. I grew up in Detroit in the 60's when the highway speed limit was 75 and everyone drove 85, more or less like now, so I have been driving at speeds around 80 or so for 32 years, when speed limits permit. I have been up to 135 in a Cadillac ambulance back in the 60's (as navigator not as driver) so I know what that is like. Even the Frontier will top 100 without effort, and when pulling out to pass in traffic on I-17, I pay more attention to other traffic, distances in front and rear, and opportunities to get back in line -- the total dynamic. My actual speed is irrelevant. I don't go faster than circumstances warrant, but I don't limit my speed arbitrarily either. If I did, I would never pass anyone no matter how fast they were going. I just don't pass with my head down, so looking at the speedo is not an option.
Having driven frequently enough and for hours on end on the Autobahn at 105mph, top speed in the Passat at the time, high speed is not strange to me, and at 105, frankly I was a slow poke. When you and everyone else around you are doing the same speed, the sensation is no different than doing 60 here at home. So having experience at speed, I don't give it a focused attention when passing. It is a familiar venue. On a two-lane where I am opposing traffic, I am even less inclined to dawdle.
And the guy is right I guess, the discussion was about truck tires.
Having driven frequently enough and for hours on end on the Autobahn at 105mph, top speed in the Passat at the time, high speed is not strange to me, and at 105, frankly I was a slow poke. When you and everyone else around you are doing the same speed, the sensation is no different than doing 60 here at home. So having experience at speed, I don't give it a focused attention when passing. It is a familiar venue. On a two-lane where I am opposing traffic, I am even less inclined to dawdle.
And the guy is right I guess, the discussion was about truck tires.
R_Lefebvre
Expedition Leader
I'm mostly with you, as long as you're in a proper car and circumstances warrant it. Just don't do it up here in Ontario. 50km/h (30mph) over, and they take your car on the spot.
