Re: [AMC-List] Rear Axle ratio choices, a question

["Swygert, Francis G MSgt 436 CES/CECM" <Francis.Swygert3@xxxxxxxxxxxx>]

I adjusted tire size to get the cruise speed/rpm I wanted. I run a
205/70R15, which makes a big difference, The AW4 OD is also 0.70:1,
which also helps. I run just under 2400 rpm (2381 according to the
calculator at www.4lo.com, using 26.3 as tire diameter). I didn't state
the OD ratio, not sure if I mentioned tire size either. I think those
are the missing parts of the puzzle! 

The logic for getting better mileage with the 3.55 than the 3.08 rear
gears is simple -- the engine wasn't operating in it's most efficient
rpm range until it hit 65 mph or more in OD (1800-2500 rpm seems to be
the best range to operate the AMC six with a stock or near stock cam,
though I know that might be an over generalization). I could have just
rarely used OD, but then take off suffered due to the low torque
multiplication of the AW4 trans. You're correct, by the way, in stating
it's more closely related to some of the newer auto trannys. It's an
early "high efficiency" design. It needs a lower first gear for take-off
than older designs that supply more torque multiplication. My car would
take off okay with the 3.08 gears, but if it had been loaded down it
wouldn't have. It's much snappier with the 3.55 and doesn't stay in
first gear long (but it's still required -- 2nd gear take-offs are a bit
slow) and I can use all four gears. Even with the 3.55 gears I don't
shift into fourth (OD) until 50 mph. If I'm cruising on relatively level
ground at 45 mph I'll go ahead and shift into OD, but much under I just
leave it in third. All this with the 3.55 gears. With the 3.08 there was
little point in shifting into OD until 55-60 mph, and at take off you
could tell the engine was working (though not what I'd cal laboring) to
get the car moving. With the engine stressed less and cruising in it's
flattest torque range, it gets better mileage. I've found the same thing
true in my J-10, though I don't have a tach connected yet. It has 2.73
gears and a non-feedback BBD installed (it was changed by previous
owner). With the manual four speed I just treat fourth gear like OD. I
only have to use low range if taking off a relatively steep incline with
a load. No load works fine in high. 

A lot depends on the transmission ratios, tire size, and engine build.
With the stock cam 1800-2500 seems to be the best torque range of the
AMC six. It should get the best mileage when operated in that range. I'm
running a NAPA "Econo Power" cam (which I believe is made by Crane) with
just a little more lift and duration than a stock 4.0L cam. It acts
pretty much like a stock cam until 2500 rpm, where it starts producing
noticeably more power, and sucking noticeably more gas. I've noticed
pretty much the same with carbed AMC sixes, but you've got more
experience than I do! 

-----------------
Date: Sunday, November 26, 2006 11:32 PM
From: John Elle <johnelle@xxxxxxxxx>

Frank.
I tried running a 3.08 gear in mine with the torquey 4.6L stroker and
AW4. It cruised at 70-75,
Snip
Assume for comparison purposes, Az State Speed limit of 75 mph.
This would calculate out (using p205-70-14 Tires) at 3068 rpm
Snip
but stressed the engine enough that gas mileage dropped considerably
compared to the T-96 w/)D and 3.31 rear gears.
Snip
Assuming the O/D was a typical .75 the final drive ratio would then be
equivalent to 2.48:1 rear axle. Thus with the T-96 and OD using the same
size tires rpm would be 2493 rpm. This would be a decrease of 575 rpm
at 75mph. A decrease of 575 rpm should carry over into an increase of
fuel economy. That is the theory behind overdrive. Decrease rpm to
fuel economy.
<snip>
I guess what I am trying to understand is the logic that says a 3.55:1
rear axle is going to give better fuel economy than a 3.08 rear axle.
Neither my own personal experience or logic makes much sense of that
unless I missed something completely.

Snip
The AW4 converter won't slip and allow the
rpm that the TF converter does before the car starts moving, so first
could end up feeling like a lot less gear than you have now.
Snip
I did not know that! But then again, that transmission must be related
to the one in my Town and Country and one more replacement for
that one and the car will get turned into a Chevy!

John.

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