I am considering purchasing a new ford ranger pickup (4x4 supercab
4.0L engine).  I also own a 4500lb boat.

the ford ranger web site lists the maximum loaded trailer weight
capacity for the manual transmission vs the automatic transmission at
significantly different values.

3100lbs towing capacity for the manual.
5580lbs towing capacity for the automatic.

here is the question:
why such a significant difference in towing capacity?
followup:
will towing a 4500lb trailer really damage a manual trans vs an auto?  Why?

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Clarification of Question by halg-ga on 17 Jan 2006 21:07 PST

Thanks for the comments.

I agree that an automatic transmission does not depend on the driver. 
Therefore, it poses less risk to the manufacturer.  This is because an
inexperienced or careless driver may 'smoke' the clutch while towing
or use an inappropriate gear.

Both vehicles (the auto & manual) have the same gears, rear & front
axle strength, etc.  The only difference appears to be the
transmission.

Provided, the driver of a manual transmission shifts at appropriate
times just as an automatic transmission would, is there an inherent
weakness in the manual transmission compared to an automatic?

The automatic is 'automatically' aware of the strain on the engine.

With the manual gearbox, everything depends on the driver, if they
don't shift gears at the right time they'll stuff the engine.

The manufacturers reckon that the chances of the driver not getting
the right gear are pretty high, and are not prepared to warranty the
engine, or and this is significant, the safety of the vehicle.

There is also the matter of deceleration, the chances of a driver
using the engine as a brake is pretty high, eg: crunch down a gear
rather than brake.
I would imagine that the trailer has its own braking system comnnected
to the pickup's brake system, ... could be nasty ...

I suspect that it would not be legal to tow 4,500 lbs on a manual.

I'd go with product liability and some inherent strength in the automatic design:

Manufacturers today are recommending the automatic over the manual
shift. Not only are they stronger and more reliable today, but they
are "smarter". In today's world they have special tow/haul shift
modes. Some automatics can actually LEARN to shift and react to the
driver and the conditions. The manual transmission today is also
greatly improved, with 6 forward gears and all-gear synchronizers now
commonplace. It really is your choice, depending on availability. Just
remember one important thing: A clutch and your LEG have no warranty.
Almost all manufacturers offer service plans up to 6 years and 100k
miles.

http://www.boatus.com/trailerclub/upkeep/trucks.asp

If I was purchasing a Ford truck, I would have to ask myself a
question.  What is the truck going to be used for?  If towing is the
answer then I would most definitely buy a truck that would tow a
little more than my intentions are. The reason for that is you will
find yourself overloading the truck.  That always happens, i see it
everyday.  As a transmission technician I will tell you that if you
want to tow a boat, I would recommend the 4.6, 3 valve F150.  The
second choice would be the F250 with 6.0 liter and the 6 speed trans. 
That is the most impressive package I have seen in a long time. The
durability of the F150 will impress you, but the F250 with the 6.0 and
the 6 speed automatic trans will make you say wow, over and over.