Ever had a 10 speed bike?  Put it in high gear and try to take off. Now,
put it in low gear and take off.

I see where "Taxbert" <Taxbert@hotmail.com> contributed:

::sigh::

One of the criteria used to set tow ratings is accel/climbing ability,
and gearing makes a difference.

What harm?  The most likely harm from running at or over the weight
limit would be tranny slippage/damage, and/or overheating.

Here is some general advice on two ratings. Note especially #7:

===================
It is my firmly-held opinion that  - on average - manufacturers'
maximum tow ratings are optimistic and presume near-ideal
conditions and a fair amount of operator skill. I continue to
recommend that people buying equipment to tow (or be towed) try
to stay well *under* the maximum. I like a 75% target, on the
basis that it provides a safety/reliability/comfort cushion for
steep grades, bad roads, and emergency maneuvers.  In recent
years as the bulk of traffic has moved over to the interstate
system, I've become increasingly concerned with the number of
people who sincerely believe it is OK to drag their long long
tri-axle trailer (at 125% of tow rating) at 75 mph with their
leetl pick-em-up.

    [There is even the Hensley hitch character going about    
    like a side show barker with his video showing an    
    Intrepid hauling a 9,600 lb trailer! This is FIVE TIMES
    the tow rating, ladies & gents! The product is good but
    the marketing is near-fraudulent.]

Bottom line:  there's a great deal more to tow ratings than
power.  To get some perspective on that, consider the average
over-the-road truck. Typically 400 horsepower or less to haul
80,000 pounds.  Do your own math.

And consider this reasoning for a 75% recomendtion:

There is pretty general agreement that it is unwise and possibly
dangerous to EXCEED maximum ratings, but many of us with long
trailering experience have found that tow rating information is
often misleading - or at least not applicable to everyone.

1. The tow rating is ALWAYS a maximum figure, and is as large as the
manufacturer dares make it. They hope you will buy their stuff for
towing.  Their rating may or may not be right for you.  Every maker
has his own methods of setting tow ratings. Some are conservative
and some are ludicrously over-stated (many Jeep Cherokees were rated
at 5,000 lbs, and IMO are hopelessly overloaded at that figure).

2. MOST tow ratings (nothing personal or specific vs your brand)
do not allow for long steep grades - up or (especially) down.

3. Most tow ratings make no allowance for bad road conditions.

4. Most tow ratings are accompanied by asterisks that call attention
to special equipment "required". Your rig may not have those features.

5. Most tow ratings make no allowance whatever for emergency
maneuvers.  I assure you your vehicle WILL NOT turn or stop as
fast or as safely with the maximum load as it will with a lot less.
The difference can be dramatic. Don't believe me?  Try a few tactics
in a large parking lot.

6. Vehicle tow ratings make no allowance for the DRIVER'S "tow
rating".  No insult intended, but if you have to ask how much your
rig will tow, you have neither the experience or the knowledge to
handle the maximum load safely.  IMO.

7. One of the most-overlooked factors in safe towing is a COMBINED
maximum (GCWR = Gross Combined Weight Rating, often only found in
a towing guide) that dictates a much lighter-than-maximum TOWED load.
Most of the weight of cargo & passengers in the tow vehicle must be
deducted from the permissible towed load to find the true rating.
Some towing guides appear to gloss over this issue because the
marketing types want to put the best possible face on their product.

8. What is reasonably safe and comfortable at 45 mph may well be a
lethal weapon at 75.  Tow ratings, IMO, do NOT reflect any respect for
this hazard.  

9. An internal combustion engine loses about 2 1/2 to 3% efficiency
per thousand feet.  You can easily lose 15-18% in the mountains
unless you have a turbocharger or supercharger.

10.  Regardless of weight ratings, SUV's and "1/2'-ton" pickups are
mostly useless for serious towing.  They will handle pop-ups and even
some of the short "lite" trailers - and will haul yer big one out to
the lake if you are careful.  But competent handling of a large TT
requires a long wheelbase & short overhang.  

Someone once wrote: "You can tow anything with anything - the question
is how far, how safely?"

BOTTOM LINE:  IF you trust the experienced trailerists who have been
there and done that and don't want to go back, you will not exceed
about 75% of the rated maximum.  The number is of course not writ by
the finger of God on a stone tablet - it is merely an indication that
you should stay well below the manufacturer's maximum allowance if you
want a safe, comfortable trip.  Some say the figure ought to be as low
as 50 or 60%.  But except for a few macho braggarts, most experienced
folks agree in principle if not detail with these concerns. For
example, go to http://www.popuptimes.com/archives/75rule.asp
   
Will Sill - rv traveling for half a century
The Curmudgeon of Sill Hill

Hi,
Basically the gears in the differential are designed to work with a
certain load.  You can ruin the differential and/or the transmission.

Towing at or above the maximum tow ratings leaves no extra margin for
error.  It causes more wear and tear on the tow vehicle and sometimes
the driver.  Towing at 75 - 80 percent of the tow ratings is easier on
the vehicle and probably safer and easier on the driver as well.  You
should find out the GCWR of your tow vehicle.  Then load the tow vehicle
up as if you were ready to travel, then get it weighed.  Subtract that
weighed number from the GCWR and the number you get is the maximum fully
loaded trailer weight you should tow.

Take care and Happy Campin...

A tow rating a 7500 lbs with a burb with a 5.3 and 3.73 gear is
wishfull thinking indeed. You might tow that much "okay" if you like
using second gear a lot and getting rotten MPG too. About 4 or 5K tops
here and with a 4.10 maybe 6k or so. The 5.3 has impressive HP rating
but it lack torque below 3000 rpm where most towing is done (it peaks
at 4000 RPM) and it s not overly abundant with torque above 3000
either. The old 5.7 Vortec is a better tow motor than the 5.3 is and
has a lot more torque below 3000 RPM for towing with. (it peaks at
2800 RPM too) Some will bash me for being overally conservative but I
can find not valid science for some of detriots current tow ratings
other than for sales needs. I do not consider the noraml usage of
second gear on highway grades as a acceptable tow combo though some
do. There was a time when a 3.73 was considered a good towing ratio
when used with 350's and 29  tires or so but now newer SUV's have high
HP ratings but little low speed torque and even taller tires yet. As a
example a old 454 with a 3.73 rated at about 100HP less than a modern
5.3 would BADLY outclass a 5.3 with a 4.10 towing because it has gobs
of low speed torque. WIth a 5.3 you need at least a 4.10 to do any
seriuos towing and even a 4.56 if you have your heart set on 8k or so.
Sure there might be some that will pipe up and say that it tows 8K
well but they have very low standards for towing performance if they
belive that it is a good combo. Below is a link to some towing info
you might find usefull

 http://forum.snoman.com/viewforum.php?f=21
-----------------
The SnoMan
www.thesnoman.com

<<<SNIP ALL>>> because this is a long response...

The first issue is towing capacity. Remember that the manufacturer's TC is
for a BASE vehicle with a 154 lb "standard" driver, no options except those
required for towing (not even "forced" options that are installed at the
factory on all vehicles), no passengers, no cargo, no tongue load and in
some cases only 3/4 tank of fuel. They also assume travel at or near sea
level, on level roads that are smooth and in good good condition.

The 2002 Chev towing guide gives the following GCWR's for the 5.3L w/ AT:
   3.42 rear = 12,000 lb, 3.73 rear = 13,000 lb, 4.10 rear = 14,000 lb

Based on the fact that my 2001 Suburban (equipped with rear seat, rear
heat/air, sunroof, roof rack, etc) weighs in at 6,000 lb with two adults and
our "normal" travel stuff but not including luggage, it's obvious that 8,000
lb is not a realistic number. The first calculation (GCWR minus actual
weight) gives a max TC 6,000-7,000-8,000 lb each. Allowing for 12-14% tongue
weight added to the Sub (added to actual truck weight, subtracted from GCWR)
gives a more realistic max TC of 5,160-6,020-6,680 lb each. Those numbers
are max TC and put the rig at GCWR. Many experienced (safe) trailerists
prefer to tow at 75-85% of max TC. Assuming no other differences in the
trucks, you "should" be able to re-gear but only Chev can give you a real
answer.

Now to the issue of "what harm"???

In the "Coach & Chassis" section of the Nov 2002 Motorhome magazine, Wes
Caughlan responds to a reader's plea for a blessing on his attempt to
increase the GVWR of his MH by adding spring leaves and tires with higher
load rating. The author explains that GVWR is based on far more than the
spring and tire capacities then adds some information from Eaton Corporation
(a major manufacturer of power transmission equipment). Without the benefit
of permission from the author or the magazine, I quote from his response:

"Then there is the issue of longevity. According to Eaton Corporation, tests
have shown that a 9 percent increase in gross vehicle weight over the gvwr
will reduce the life of the gears and other axle parts by a whopping 40
percent. On the good side, a 9 percent reduction in gross vehicle weight
from the gvwr will increase the life of gears and other axle parts by an
even more whopping 60 percent. That's why everything lasts longer when a
motorhome weighs less than the gvwr"

After many years in machine design and testing, I've learned that the life
expectancy of mechanical components is not a fixed number. You are not
guaranteed XXXXXX miles before various parts of your machine fail. Some
parts that are well within their load limits can and do fail prematurely but
the averages are still in favor of avoiding overload. Research conducted by
companies like Eaton looks at large amounts of data and it appears their
conclusion is that overloaded components will fail earlier than
non-overloaded components regardless of the potential or actual life of a
given part.

While the article refers to a question on motorhomes, it applies to all
vehicles and especially those used for towing. I also believe that this
research can be interpreted to conclude that any towing reduces the life
expectancy of drive train components. These loads have their greatest effect
during acceleration, deceleration and braking. In reality you are always
accelerating. Acceleration is the result of force applied to a physical
body. From the engineering and physics of the problem, you must apply force
from the drive train to keep the vehicle moving at a constant speed. You're
applying positive acceleration to offset the deceleration (negative
acceleration) due to wind drag, rolling resistance, hills, etc. All things
considered, it's better to keep loads under the manufacturer's ratings.

CruzMastr (Iowa)

I would like to thank everyone for their responses.  I do have a couple of
comments.

First, why do some people assume that I am an inexperienced towed?  Becaused
I asked a question?  I have been towing a 5,000 lb travel trailer up and
down the east coast for several years and have logged many miles.  I want to
upgrade to a larger trailer and am trying to figure out if I need to upgrade
my vehicle also.  Just because I am not an automotive or mechanical
engineer, and don't talk towing capcaity around the campfire, does not make
me an inexperienced tower (hence the question, what harm can I do to my
vehicle).  BTW, a mechanic friend of mine says I can pull the 8,000 lbs with
no problem.

I am generally a consertative guy and the 75% rule seems like a good rule
follow.  Now I am going to have to decide if I want a new trailer and a new
vehicle.

What are people's thoughts on a Suburban with a larger engine that has a
towing capacity of 12,000 lbs.  (I would prefer a suburban over a pickup).