Re: Tow vehicle

["Linda B. Merims" <lbm@ici.net>]

Merryben@aol.com said:
>
> Linda B. Merims <lbm@ici.net> said:
>>The
>>downside, of course, is that 4:10 trucks have *lousy* mileage.
>>So medium might be a reasonable compromise.
>
>
>I could be wrong but I thought 4/10 was the lower.  I have a
>Dodge diesel with a 4/10 rear end and 4 wheel drive. It is a
>supercab and my trailer is a Featherlite with LQ that weighs
>about 10,000.  The whole rig goes close to 10 tons.  I came back
>from a ride this weekend and my mileage was 16.7 miles per
>gallon.....mb

What Merryben is pointing out is that the real world is more
complicated than the simple model I put forward.

Two different things are going on here simulatenously.

What I was saying is that 4.10:1 is the rear axle ratio with the
most torque--the drive shaft turns 4.10 times for every 1 turn
of the rear axle.  It is like a "low" gear.  Because the rear
axle/wheels turn the fewest times for each drive shaft/engine
RPM, it gets the poorest mileage.

So, if you unhitched your trailer, and then could swap out
your rear axle, your truck would get better mileage *unloaded*
with a 3.73:1 "medium" rear axle or a 3.2:1 "high" rear axle
than it would get *unloaded* with your 4.10:1.

Now the tricky part is answering the question "how much torque
is enough?"

You pull a pretty big trailer with a pretty big truck (that has
a high mileage efficient diesel engine).  It is conceivable
that one could have a big engine coupled to a lesser torque
(3.73:1 or 3.2:1) rear axle hooked to a big, heavy trailer.
(I've deliberately left the transmission out of this scenario--it
figures in significantly but complicates the picture unnecessarily.)
The weight of the trailer may be too much for the engine/rear axle
ratio.  In this case, the engine is going to have to work
very hard to accelerate, constantly downshifting and the real
mileage may be *worse* than what a 4.10:1 would produce, and the
engine is going to tend to overheat and have a short life.

But Kristen's original post specified:

  "I'm shopping for a used tow vehicle for a 2 horse trailer."

On a highway on normal USDOT grades, 4.10:1 seems like
overkill for this kind of lightweight towing operation.

But it is all a matter of balancing the equation to the job at
hand.  If the truck she gets has a puny 305 engine, maybe 4.10:1
is what she needs.  (What a way to order a truck--305 with
a 4.10:1!!)  But if she gets a most-common medium V8 like a
350 or so, 4.10:1 power is going to be unnecessary most of the time,
leaving wasted gas and excess power on the highway.

But it could come in real handy going up that steep dirt road
between the highway and the campsite!

Linda B. Merims
lbm@ici.net
Massachusetts, USA