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

["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
Going to a 3.55 gear improved performance and mileage. I dropped
about 4 mpg with the 3.08, gained 2 mpg back with the 3.55. I think the
3.31 gears are probably the best mileage gears for my combo, but the
3.55 is a good compromise.
Snip
This statement does not ring 100% true. At least according to overdrive
theory. 
A 3.55 final drive should give and engine rpm of about 3526 rpm at 75
mph.
 This would be an increase of 458 rpm over the 3.08 rear axle and an
 increase of 1033 rpm over the 3.31 final drive in over drive. Fuel
mileage
 it would seem to me would go down the toilet using a 3.55 final drive.
Performance may experience an increase providing that the engine has
 the rpm capability to work efficiently with the short rear axle gears.

 
My modified 258 I-6, Iskenderian cam and 9:1 compression initially 
with the stock BBD 2bbl and full 1980 factory smog easily pulled the 
2.53:1 final drive w/P245-60-14 tires up to the Arizona speed limit of 
75 mph. This package calibrated out to 29.76 mph/1000 rpm. Thus at 
75 mph the engine ran at 2520 rpm. This engine using the BBD Carburetor 
Rear Wheel Dyno-ed out at 85 hp at 2800 rpm with a peak of 90 hp
between 3100 and 3500 rpm and fell off to about 85 hp at 3600 rpm. 
The horse power curve can be directly traced to the small cfm rating of
the Carter BBD 2BBL carburetor of about 195 cfm. 
The installation of the port fuel injection extended the rpm range of
the
 engine well past 4500 rpm before performance started to fall off. 
I do not have a dyno  test of the completed fuel injected engine but
 crude tests and calculations  seem to add about 20 additional rear 
wheel hp  but the rpm range has been improved to where wide open 
throttle 2/3 shifts are set for 4500 rpm or about 75 mph. 
Compared to my bone stock 1980 AMX which too can cruise easily enough
with the factory stock 2.53:1 rear axle at the 75 mph speed limit, all
bets 
are off at much over that speed. The engine falls on it's face at about
3400 rpm
and the ability to pull tall hills rests mostly with in your
imagination. 
However it regularly gets 24 mpg and better driven at speed limits on 
long trips. This engine with P205-70-14 tires calculate out to 
29.76 mph/1000 rpm. I can see if a given engine is more than 
a bit anemic, that performance would suffer quite heavily if driven
 in O/D regularly. Also fuel economy would go down the toilet 
too as the tendency would be to flog it a bit to get it to move and 
nothing would happen other than sucking gasoline at a copious rate.
 But that same engine would probably not be overly happy running
 at rpm levels that can be seen on interstate or other high speed 
roads. It is just that my experience with the AMC I-6 engines I 
have played with have the capability of running well with rather 
tall final drives whether from the factory as in my 1980 Spirit with
 the 2.53:1 rear axles or induced through the installation of an 
overdrive type of transmission. 
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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