Gas, Diesel, Gears and Towing

Gross combined weight rating is a manufacturers number used to protect the drive train, which helps keep warranty costs lower. If you are close on this number, or just over, and your truck is newer/low mileage, your engine, transmission, and all things moving back to rear wheels are likely to be able to sustain this slight overload for quite a while. On an older/high mileage truck you may have failure, while staying just under this number.

Since the gearing changes the workload, 4.10 will have a higher combined towing rating than say 3.73 in the same exact truck.

Jerry

So to sum this all up.....Most of a tow vehicles numbers are very important. Axle ratings, payload etc. But GCVR doesn't mean crapola to some people if you're over unless of course you're talking to the other people, then it's a big deal. Kind of like slider hitches. You don't need one until you need one but you may never need one until that one time you might need one. It's all clear as mud now! Moral of the story....Stay within Specs, err on the side of caution, enjoy life to the fullest.

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.

The law--which generally is applicable to commercial haulers rather than Joe Fifth Wheel--uses the metric of axle weight ratings.

Also look up federal bridge laws, this says you get up to 20k lbs per axle 34,000 per tandem also there is a 37k lb for two axles. Difference being HOW far apart the axles are, to "bridge" or spread the load on the road bed. This is an engineer based design limit of the road.
Even if a manufacture rated an axel at 30k lbs, you can only out 20k lbs and legally run down the road. There are a couple of exceptions, but for those pulling towing carrying in the RV world, those exceptions will not work. At least from this forum standpoint. In type A MH bus chassis relm, you can get to 24,000 lbs on an axle.
I've been pulled over at 150% of my dump trucks manufacture GVWR, No overload ticket as I was under the bridge law amount. Altho about 1200lbs iirc my paid for license. Got a 10 day up license by a ton, sent on my way down the road.
aLL Leo's in wieght divisions care about is you having paid the tax, so damage you are doing to the road is paid for via registration.
Marty

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.

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.

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.

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



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

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

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.