Good Sam Club Open Roads Forum: Towing: Gas, Diesel, Gears and Towing
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 > Gas, Diesel, Gears and Towing

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opnspaces

San Diego Ca

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Posted: 05/09/20 12:07pm Link  |  Quote  |  Print  |  Notify Moderator

If you have time, read the sticky about federal weight limits at the top of the Tow Vehicle forum. Pay particular attention to the posts from Wadcutter. Of anybody on here, he has at least admitted to being a bit of an authority on the subject.

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KD4UPL

Swoope, VA

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Posted: 05/09/20 12:30pm Link  |  Quote  |  Print  |  Notify Moderator

It also has to do with hitch rating, payload rating, tire rating, spring rating, transmission cooling capacity, axle rating, and the marketing department.
There's no formula for X amount of HP or torque with X gear ration can tow X amount of weight.

RobWNY

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Posted: 05/09/20 01:08pm Link  |  Quote  |  Print  |  Notify Moderator

I'm not trying to open any kind of can filled with worms lol. Just trying to learn. Some say you should never exceed this or that rating while others say you'll be ok because there's always a cushion that manufacturers factor in. While others really don't care. I'm in the camp of wanting to learn so I can make an informed decision. I was a cop for 30 years. Never once in my career did I look at door stickers to make sure vehicles were within weight ratings at accident scenes. Even commercial weight cops didn't get involved. They only had their eyes on Tractor Trailers. What I do know though is I've never met an attorney that wasn't trying every angle he or she could come up with to get more money awarded in a law suit regarding accidents. If finding out a truck and RV are over the GCWR shown by the manufacturer, it will be an angle they would pursue.


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Lynnmor

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Posted: 05/09/20 01:14pm Link  |  Quote  |  Print  |  Notify Moderator

Gearing is selected using the operating speed of the engine. Usually a diesel develops its maximum power at a much lower RPM than a gas engine. Also the maximum safe RPM of a diesel is usually much lower. Yes, the manufacturers know what they are doing when selecting gear ratios.





valhalla360

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Posted: 05/09/20 01:25pm Link  |  Quote  |  Print  |  Notify Moderator

Torque at the engine crank shaft is modified by the gearing...but not just the rear diff but also the transmission gear ratio. Depending on which gear you are in, this translates to a torque at the wheels. A lower transmission gear will result in higher torque at the wheels.

Diesel engines operate at roughly twice the compression ratio, so they can generate a lot more torque at the crank shaft...especially at low RPM. Therefore, even though truck C has a higher geared rear end (lower numerically), it can still get more torque to the wheels (assuming the same transmission gearing).

With modern 8-10 speed transmissions, the rear diff has less impact unless you are pushing the truck to the limits, as there are more options for the transmission to pick the best gear to generate the required torque at the wheels (so you are seeing fewer rear diff options and a higher gear ratio more often because the transmission can adjust to what is needed)

Now torque just determines if you can climb the hill. Put a low enough gear in and you could drag the QEII up Pike's Peak...just not very fast because RPM has a maximum value.

HP is what determines how fast you can climb the hill (assuming you have enough torque in the right gear to keep it moving). This creates a myth that a 300hp diesel will out tow a 400hp gas engine. HP = Torque * RPM.
- With the diesel, it generates a lot of torque but has a much lower peak RPM.
- With the gas, it's happy to turn much higher RPM. You might hit peak HP around 4000rpm. People used to cruising around in trucks with empty beds aren't used to running at such high RPM and assuming (falsely) that it will damage the engine. As long as there isn't a preexisting problem, modern gas engines will not be harmed. It will tend to be a bit noisy, so if you frequently climb long steep grades, aesthetically it's displeasing.

One deviation from this for climbing mountains is naturally aspirated engines (diesel or gas) are at a sever disadvantage compared to turbo engines (diesel or gas). You lose about 3-4% of HP for every 1000ft of altitude due to thinner air. So at 10000ft, you are down 30-40% on power with a NA engine. A turbo engine on the other hand force feeds air resulting in minimal power loss at altitude.

The GVWR will generally be the same for an otherwise identically spec'd truck. But since the diesel weighs so much more, the PAYLOAD will typically be lower. Often, it is the payload that limits the trailer size, particularly for 5th wheels. 20% of a 15000lb 5er, is 3000lb pin weight...add in passengers, fire wood and other gear and a lot of 3/4ton trucks are over payload even if the tow rating is OK.

As far as stopping and handling, really doesn't make a lot of difference but make sure you are comparing apples to apples (Base 3/4 ton 4 door vs max payload 1 ton dually standard cab are not comparable). Once otherwise identical, it's about brakes and suspension. There is also a registration fee aspect that sometimes comes into play. Above certain GVWR, may result in different registration fees.

Now if you start looking at which option provides the ultimate highest CGVWR (combined total truck and trailer), it will usually be a diesel with a low geared (high numberically) rear diff as that gets the most torque to the drive wheels.


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RobWNY

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Posted: 05/09/20 01:39pm Link  |  Quote  |  Print  |  Notify Moderator

Thank you everyone for your input. I'm learning! But there better not be a test at the end of this lesson [emoticon]

BenK

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Posted: 05/09/20 01:43pm Link  |  Quote  |  Print  |  Notify Moderator

Simply stated, without the details in the rat-holes...

If it takes 150 HP to move our TV & Trailer 65 MPH. No head wind, flat, etc...then chose a gear that will keep the ICE RPMs at that 150HP range

[image]

That is at a constant speed with no headwind, flats, etc

From a dead stop accelerating up to that speed, it will take more HP and dependent on how quickly you wish to get to that 75MPH

Say it will take 300 HP to get it going and accelerate at the rate you wish. Then you must keep the ICE spinning at whatever RPM necessary to keep it at 300 HP. That is what gearing is for and once 75MPH attained, you will need less HP and again, gearing to drop the ICE RPMs down to 150HP

If you wish to accelerate quicker and/or cruise at a higher speed, then match the HP required to the RPMs that ICE's curve says it will need to get you that HP

Makes no matter gasoline or diesel...the difference is that one has more BTU's per unit measure and does the job a bit differently.

Again, why the close ratio gear boxes with more gears helps in this regard...keeping the ICE in the good portion of the HP/Torque curves


-Ben Picture of my rig
1996 GMC SLT Suburban 3/4 ton K3500/7.4L/4:1/+150Kmiles orig owner...
1980 Chevy Silverado C10/long bed/"BUILT" 5.7L/3:73/1 ton helper springs/+329Kmiles, bought it from dad...
1998 Mazda B2500 (1/2 ton) pickup, 2nd owner...
Praise Dyno Brake equiped and all have "nose bleed" braking!
Previous trucks/offroaders: 40's Jeep restored in mid 60's / 69 DuneBuggy (approx +1K lb: VW pan/200hpCorvair: eng, cam, dual carb'w velocity stacks'n 18" runners, 4spd transaxle) made myself from ground up / 1970 Toyota FJ40 / 1973 K5 Blazer (2dr Tahoe, 1 ton axles front/rear, +255K miles when sold it)...
Sold the boat (looking for another): Trophy with twin 150's...
51 cylinders in household, what's yours?...

troubledwaters

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Posted: 05/10/20 07:03am Link  |  Quote  |  Print  |  Notify Moderator

Think about it for a minute, how is an attorney going to prove you were over any of your ratings if it wasn't weighed at the scene of the accident? And why bother when they only have to prove you failed to stop in time.

rhagfo

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Posted: 05/10/20 07:15am Link  |  Quote  |  Print  |  Notify Moderator

opnspaces wrote:

That's a question that none of us can answer. How the manufacturers arrive at ratings seem to be a closely guarded secret.

RobWNY wrote:

you get in an accident, a smart attorney could sue you for being over the stated GCWR

I don't understand how torque and HP plays a roll in an accident.


Now you're really opening a can of worms. There are many discussions on here about whether the GCWR and or towing rating are legal ratings that can be admitted in a court case. Ultimately nobody on here can answer that. Well it's possible that someone on here can answer these questions, to date they have been mute on the subject.


This may not be an actual legal issue, BUT the fact in every owners manual states DO NOT EXCEED any of the weight limits, there is they can have a case. I will leave at that.


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JIMNLIN

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Posted: 05/10/20 01:35pm Link  |  Quote  |  Print  |  Notify Moderator

op wrote:

What I do know though is I've never met an attorney that wasn't trying every angle he or she could come up with to get more money awarded in a law suit regarding accidents. If finding out a truck and RV are over the GCWR shown by the manufacturer, it will be an angle they would pursue.

Even if they were ignorant enough to try it won't go anywhere. Biggest reason trucks aren't assigned a GCWR...just FAWR/RAWR/GVWR. Truck mfg may assign any of the above numbers they choose.

GUIDELINES FOR CALCULATING GVWR from FHWA
**"Vehicle manufacturers may determine the GVWR of a vehicle by calculating the maximum load rating of the vehicles axles, tires, brakes, frame, suspension and wheels in order to come up with a GVWR rating. Also, a manufacturer may limit the GVWR if so desired. So they could have the componentry to rate higher but choose a lower rating at their discretion for excise tax purposes.**

GCWR ?? NHTSA website some years back.
Gross combination weight rating (GCWR) is the greater of:

**(1) A value specified by the manufacturer of the power unit, if such value is displayed on the Federal Motor Vehicle Safety Standard (FMVSS) certification label required by the National Highway Traffic Safety Administration, or

(2) The sum of the gross vehicle weight ratings (GVWRs) or the gross vehicle weights (GVWs) of the power unit and the towed unit(s), or any combination thereof, that produces the highest value.**

As we know our trucks aren't assigned a GCWR so we go to (2) paragraph.

DOT considers the sum of the trucks GAWRs can safely be used as its GVW or a GVWR.
NHTSA says this about components of the GAWR:
**"Gross Axle Weight Rating is the rated load-carrying capacity of an individual axle and wheel assembly. (It represents the load that may be steadily sustained by the components in the system; i.e., tires, rims, hubs, bearing, axles, brakes, suspension, sub frame, etc. with the GAWR limited by the components with the lowest working rating".**


Coming from the commercial side using mostly one ton DRW trucks and heavy GN trailers we don't get sued or ticketed by exceeding the truck makers GVWR..... and of course a GCWR as its not on the trucks certification placard so no ticketds or lawsuits there either. WE can be sued/ticketed by exceeding a declared GCW or the vehicles steer/drive or trailer axle/tire load ratings.
I doubt the same size truck pulling the same weight rv trailer is any different.


A GCWR can be changed by a simple gear change. Or tranny change. Or a engine change. Or the same engine with a 2 barrel carb had a different GCWR than the same engine with a 4 barrel carb. And with some older trucks even a wheel/tire size would affect a mfg GCWR.

The truck mfg tow rating is a good number to shoot for however I sure wouldn't exceed the GAWRs....mainly the RAWR as its carrying most if not all of a trailer hitch load and other gear in the bed.


"good judgment comes from experience, and a lot of that comes from bad judgment" ............ Will Rogers

'03 2500 QC Dodge/Cummins HO 3.73 6 speed manual Jacobs Westach
'97 Park Avanue 28' 5er 11200 two slides

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