# Semi-Trailer Trucks Cruising at 120 MPH: Myth? **URL:** \ **Category:** Factual Questions\ **Created:** [November 9, 2001, 3:24am UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347 "2001-11-09T03:24:14Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [November 10, 2001, 3:07am UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/41 "2001-11-10T03:07:48Z")
> [@](#): > > \*Originally posted by The Ryan \* > \*\*I can understand how drafting would increase the mileage, but how would it increase the top speed? The front truck doesn’t get any benefit from drafting. \*\* Ahhh…but the crazy thing about fluid dynamics is that it can get a benefit from the drafting, by having a longer aerodynamic “tail”, and splitting the drag over it and the vehicles behind it. It’s not a big benefit, but it does exist. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [November 10, 2001, 4:06am UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/42 "2001-11-10T04:06:52Z")
**Anthracite** don’t sell short that ‘tail’ as a tail will lower the c.o.d. more then a aerodynamic head. ---
**Author:** ![tsunamisurfer](https://avatars.discourse-cdn.com/v4/letter/t/a698b9/32.png) [@tsunamisurfer](https://boards.straightdope.com/u/tsunamisurfer)\ **Post date:** [November 10, 2001, 4:21am UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/43 "2001-11-10T04:21:07Z")
> [@](#): > > \*Originally posted by Anthracite \* > \*\* > > > [@](#): > > > > \*Originally posted by The Ryan \* > > \*\*I can understand how drafting would increase the mileage, but how would it increase the top speed? The front truck doesn’t get any benefit from drafting. \*\* > > Ahhh…but the crazy thing about fluid dynamics is that it can get a benefit from the drafting, by having a longer aerodynamic “tail”, and splitting the drag over it and the vehicles behind it. It’s not a big benefit, but it does exist. \*\* My crazy uncle–as opposed to the cerebral one–once claimed that he once pulled up behind a tractor trailer at night to within 10 feet, snapped off his headlights, put the car in neutral, and said the truck’s draft was so strong it pulled his Cadillac down the road without him having to even touch the gas pedal. Come to think of it, the trucker claiming to drive at 120 mph speeds looked a hell of a lot like my crazy uncle. (Note: I say these things just to drive **Sam Stone** nuts.) ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [November 10, 2001, 6:10am UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/44 "2001-11-10T06:10:26Z")
I’m reading, and I’m not buying most of it. First of all, it is _impossible_ to be ‘pulled along’ by drafting behind a vehicle. There is no substantial ‘vacuum’ behind the vehicle - just a lot of turbulent air. And even if drafting lowered your wind resistance to zero, the rolling friction of the tires and drivetrain would still require you to add power to the system to stay there. And we’ve established that there are \*some trucks that are physically capable of hitting 120, barely, when not hauling a lot, on a flat surface. That’s a far cry from a convoy of trucks doing 120 on the open road. Remember, the convoy can’t go any faster than the slowest truck. It’s a tall tail. Never happens. ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [November 10, 2001, 6:22am UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/45 "2001-11-10T06:22:58Z")
I am at a loss to understand how the drag on a semi is greatly affected by the weight of the load in the trailer. I will agree that the truck will take longer to reach a given speed with a heavy load but air resistance will not be changed and resistance due to tire compression and friction on the axles will not be much greater will it? ---
**Author:** ![DPWhite](https://avatars.discourse-cdn.com/v4/letter/d/d26b3c/32.png) [@DPWhite](https://boards.straightdope.com/u/DPWhite)\ **Post date:** [November 10, 2001, 6:49am UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/46 "2001-11-10T06:49:27Z")
I will sometimes drive to and from SF to LA late at night. The truckers are definitely within two car lengths of each other and going about 80 mph. At speed (for me about 85 on I-5) I have never been passed by a semi. I used to have an 89 Accord, which I had tires for 130, and I did 115 in it from time to time. Remember, if you don’t have tires rated for speed, you will die at over 100 mph. So get good tires. And if your car doesn’t handle as well at speed as it does at 65, then slow the hell down. Most American cars are not designed for this and the risk is very high. ---
**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\ **Post date:** [November 10, 2001, 12:36pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/47 "2001-11-10T12:36:25Z")
> [@](#): > > \*Originally posted by gazpacho \* > \*\*I am at a loss to understand how the drag on a semi is greatly affected by the weight of the load in the trailer. I will agree that the truck will take longer to reach a given speed with a heavy load but air resistance will not be changed and resistance due to tire compression and friction on the axles will not be much greater will it? \*\* I had the same thought while reading this thread. Harkening back to my Physics days… the air resistance should be the primary speed limiter, and is unrelated to the weight of the cargo carried. You may get a few MPH difference due to tire and axle issues, but that’s about it. As I understand it, though, some trailers are designed to drop down extra tires if a heavy load is being carried, that should have a bit more effect. [WAG] I’m also thinking that having weight in the trailer will make the rig more stable at high speed, not counting turns or stopping, just straight line. Air turbulence will not be able to push the rig around with a more massive trailer load. The forces will be the same strength, acting on a much more massive object the resulting acceleration will be reduced, adding stability. Therefore, I would suspect that trailers that are heavily loaded will be **more** able to travel at high speed.[/WAG] ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [November 10, 2001, 1:49pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/48 "2001-11-10T13:49:11Z")
I missed the part about weight. **gazpacho** and **Cheesesteak** are correct, the weight has little impact at all (other than some change in rolling resisitance of the tires, and bearing friction) on the top speed. The top speed, on an equal slope surface, is a function of two primary things: Drag. Power. I agree with **Cheesesteak** that the increased weight would lend stability. ---
**Author:** ![Patricinus\_Scriblerus](https://avatars.discourse-cdn.com/v4/letter/p/f07891/32.png) [@Patricinus\_Scriblerus](https://boards.straightdope.com/u/Patricinus_Scriblerus)\ **Post date:** [November 10, 2001, 1:53pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/49 "2001-11-10T13:53:56Z")
> [@](#): > > \*Originally posted by Tony Montana \* > \*\* > > > [@](#): > > > > I’ve never seen a truck going more than 90 mph > > so if its outside your own experience,it must not be true? > Ive seen truck doing at least 100 MPH. > > > [@](#): > > > > (and there’s no highway so isolated you won’t pass SOMEONE.) > > ever driven rural hwys? Try driving from CA to Prince rupert BC. the yellow head #7 HWY in BC . \*\* The Yellow Head #7 is known as the “other trans-Canada highway” It is by no means isolated. ---
**Author:** ![tsunamisurfer](https://avatars.discourse-cdn.com/v4/letter/t/a698b9/32.png) [@tsunamisurfer](https://boards.straightdope.com/u/tsunamisurfer)\ **Post date:** [November 10, 2001, 5:00pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/50 "2001-11-10T17:00:32Z")
> [@](#): > > Originally posted by Sam Stone \* > That’s a far cry from a convoy of trucks doing 120 on the open road. Remember, the convoy can’t go any faster than the slowest truck. It’s a tall tail. Never happens. \*\* **"Never happens"???** Sam, how do you explain the eye-witness accounts and related observations of your fellow Dopers? Read the thread again. Surely this isn’t a gathering ground for pathological liars and tellers of tall tails. Also, notice my OP: “speeds _approaching_ 120 mph.” Remember, we’re not talking Edmonton and we’re not talking Canadians. We’re talking crazy American concrete cowboys hopped up on speed, sitting behind the wheel of 40-ton trucks and _occasionally_ blasting down the highways during the dead of the night at speeds approaching 120 mph. **Never happens? Never?** In America, if it **can** be done, it will be done. Amen. ---
**Author:** ![SCSimmons](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@SCSimmons](https://boards.straightdope.com/u/SCSimmons)\ **Post date:** [November 10, 2001, 6:55pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/51 "2001-11-10T18:55:44Z")
> [@](#): > > \*Originally posted by Balthisar \* > **Yeah, those old Escorts didn’t reach all that high of a speed. I had two Escorts of that vintage – an 84 and an 87 – and notice I _didn’t_ put their top speeds up in my previous post. Doing 80 felt dangerous, and scared the hell out of any passengers there may have been.** Geez, how’d you get it to go 80? My all-time fav bumper sticker I saw on a Ford Escort of that vintage: “O to 60 in 15 minutes”. Didn’t seem like too much of an exaggeration to me. 🙂 [/hijack] ---
**Author:** ![elmwood](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/elmwood/32/3060_2.png) [@elmwood](https://boards.straightdope.com/u/elmwood)\ **Post date:** [November 10, 2001, 8:55pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/52 "2001-11-10T20:55:13Z")
I used to drive in the 95 MPH range all the time during my Denver years, when I took frequent road trips to Las Cruces. On the stretches of traffic between I-25 between Raton and Las Vegas (New Mexico, that is), and Albuquerque and Cruces, there’s almost no traffic at night. Armed with a '95 Ford Contour with a V-6 Duratech engine and a Valentine One radar detector, zipping along in the 90s felt comfortable. Psychologically, though, once the speedometer topped 100, I felt uneasy. The car was stable even up to 110, but one wrong move and I’d be vaporized. Sadly, there’s few opportunities to open up like that in Florida, where the Interstates carry far more traffic than the stretches of pavement out west. Four lane, dual carriageway, limited access highways in the middle of nowhere, folks – God Bless America. ---
**Author:** ![DPWhite](https://avatars.discourse-cdn.com/v4/letter/d/d26b3c/32.png) [@DPWhite](https://boards.straightdope.com/u/DPWhite)\ **Post date:** [November 10, 2001, 9:23pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/53 "2001-11-10T21:23:05Z")
I guess something else I left out of my earlier 140 mph bragging (although I did mention that this was a BMW M3) is that I only did this on deserted straightaways (most of the trip) in broad daylight on smooth pavement in good weather conditions. In addition to having z rated tires, the brakes on an M3 are fantastic, but it takes a long distance to slow down at those speeds. This car was designed to go that fast. I also had my brother (who had never been over 90 mph in a car before) acting as my spotter. In short, this is not something you want to do in just any situation, or with any car. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [November 10, 2001, 9:30pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/54 "2001-11-10T21:30:44Z")
I can easily believe that speeds like that might be possible, under ideal conditions. But the thing is, conditions are never ideal. What if a deer happens to be crossing the road? If you go over a deer at 120 mph you are going to have a terrible time keeping control of your truck. And what about the civilian drivers? The highways are never 100% deserted. I can imagine such a thing happening once or twice with some psychopath drivers. But routinely? ---
**Author:** ![sleestak](https://avatars.discourse-cdn.com/v4/letter/s/919ad9/32.png) [@sleestak](https://boards.straightdope.com/u/sleestak)\ **Post date:** [November 10, 2001, 9:31pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/55 "2001-11-10T21:31:18Z")
Once while driving from Elephant Butte to Albuquerque I was passed by a semi doing at least 105. I was driving between 95 and 100 and he just blew by me. The trucker would get on the cb about every 5 minutes and exclaim “Man, I _LOVE_ my truck”. ---
**Author:** ![DPWhite](https://avatars.discourse-cdn.com/v4/letter/d/d26b3c/32.png) [@DPWhite](https://boards.straightdope.com/u/DPWhite)\ **Post date:** [November 10, 2001, 9:38pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/56 "2001-11-10T21:38:56Z")
One would have to be pretty psycopathic to do this on a public road in a semi. I’ve never seen one do more than about 80 to 85 as mentioned above. I’ve seen some semis weave in and out of heavy traffic at 75 (I ratted him out to the CHP cause this guy was going to kill someone, otherwise I would have let it go.) Like I said, I only did my speeding in the desert where I could see absolutely everything. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [November 10, 2001, 11:08pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/57 "2001-11-10T23:08:51Z")
But the weight is a factor if the road is going up slope and most roads do have at least some minor up’s and down’s. Possibly drainage slope would be enough to slow down a fully loaded rig traveling in excess of 100 mph. ---
**Author:** ![wolfman](https://avatars.discourse-cdn.com/v4/letter/w/a8b319/32.png) [@wolfman](https://boards.straightdope.com/u/wolfman)\ **Post date:** [November 10, 2001, 11:52pm UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/58 "2001-11-10T23:52:05Z")
Another thing to remember is the penalty for some of thoe things. Going more than 25 over the limit is more than a traffic offense, usually becomming high class misdemeanors, and possibly accountable under offenses like endangering the public saftey. Getting caught is not a matter of a couple of “here’s a couple points on the license, on you’re way, wink wink”. A trucker going 120 would likely get several days in jail, immediate suspension of license, and fines over 3 digits. There maybe a few examples of idiot truckers going that fast, but organized streches of highway where convoys hit 120 would very quickly attract the attention of some highway cops looking to put some cash in the coffers. ---
**Author:** ![tsunamisurfer](https://avatars.discourse-cdn.com/v4/letter/t/a698b9/32.png) [@tsunamisurfer](https://boards.straightdope.com/u/tsunamisurfer)\ **Post date:** [November 11, 2001, 4:38am UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/59 "2001-11-11T04:38:43Z")
> [@](#): > > \*Originally posted by DPWhite \* > \*\*One would have to be pretty psycopathic to do this on a public road in a semi. I’ve never seen one do more than about 80 to 85 as mentioned above. I’ve seen some semis weave in and out of heavy traffic at 75 (I ratted him out to the CHP cause this guy was going to kill someone, otherwise I would have let it go.) Like I said, I only did my speeding in the desert where I could see absolutely everything. \*\* Damn your lawyerly retractions and public-service advisories, DPWhite. Your first post re: driving 140 mph was an absolute work of art. I envisioned a handsome rake rocketing down the highway in a fire red sports car, one hand clutching the wheel, another holding a curvaceous blonde, while the both of you are laughing and having a blast. I could feel the wind, the excitement, the danger–and then you blow the whole damn illusion. I hate you. ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [November 11, 2001, 8:28am UTC](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347/60 "2001-11-11T08:28:01Z")
Tsunamisurfer: Well, “Never” is a pretty strong word, but I’ll stand by it. I’ll bet there has never been a CONVOY of trucks cruising the highways of the U.S. going 120 mph. A single truck with a lunatic at the wheel and no load? Possibly, on very rare occasions. But you can’t convince me a convoy ever did that. As for weight affecting top speed… It sure does. The heavier the vehicle, the more rolling friction. Rolling friction is substantial. [Previous page](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347.md?page=2) [Next page](https://boards.straightdope.com/t/semi-trailer-trucks-cruising-at-120-mph-myth/92347.md?page=4)