In an attempt to achieve a
little more precision in axle ratios, I prepared a spreadsheet showing “axle
ratios” for a wide range of ring teeth and pinion teeth. That way,
I could at least infer the likely combination of pinion and ring teeth for a
given axle ratio expressed to two digits to the right of the decimal
point. But, inconsistent roundoff practices often lead to erroneous
prediction of toothiness. Following are a few of the
examples:
Ratio
Ring teeth/Pinion
teeth
2.76:
47/17=2.7647
2.93: 44/15 or
41/14=2.9393 or 2.9286
3.15:
41/13=3.1538
3.18:
35/11=3.1818
3.23: 42/13=3.2308
3.31: 43/13 or
53/16=3.3077 or 3.3125
3.54: 46/13=3.5385 (The
’55 service manual says this is a 39 x 11 axle (3.5454 should be rounded off
to 3.55)
3.73:
41/11=3.7272
Older Chrysler 300 service
manuals sometimes include tooth count as well as the ratio rounded off to two
decimals. But, I did spot an error in the tooth count in the ’58 service
manual. It indicates a 35 x 14 gives a 3.18 ratio. Should be 35 x
11. I’ll bet the discussions on what axle ratio to install as “standard”
have gone on into the night for years as have those same conversations at
trackside. The selection of the 3.23 axle for 300K’s seems to indicate
acceleration was valued higher than top speed or economy. Sounds
reasonable.
At least some of the IBM build
tickets have information on axle ratio and whether SureGrip equipped. My
information on the coding for 1964 is incomplete, but for sure,
522=Sure-Grip. I believe the axle ratio is defined in the 530
section. My 300K ram convertible with TorqueFlite shows code 532 on the
IBM card which Gil notes may be a 2.76 or 3.23 ratio. Other information
I have indicates code 532 is a 2.76 axle with Sure-Grip. So my loaded
ram convertible came with the “high-speed” or economy axle? Go
figure. Car was sold new in Memphis, possibly for severe duty on back
country roads?
Impacts: With a 3.2308
axle, at the 4,800 RPM point where the FirePower 390’s maximum horsepower is
rated, the car would be turning 124 MPH on 28.10” OD 8.50 x 14 tires. Or
with a 2.76 axle—145 MPH. At 60 MPH, 2,318 RPM with a 3.23 axle or 1,980
RPM with a 2.76 axle.
There are lots of versions of
equations to convert driveshaft RPM to speed and I like:
MPH=0.002975 x RPM x OD/R where
RPM is driveshaft speed in revolutions per minute (engine speed less any
slippage) OD is the running outside diameter of the tire in inches (may vary
from static measurement) and R is the axle ratio.
Example: For:
RPM=2,318, OD= 28.10 and R=3.2308; MPH=59.98 on dry pavement with no
slippage and in a perfectly straight line.
If you are interested in
precision, disassemble the differential and count the teeth. If you want
to get close enough to “differentiate” between nominally-available values,
jack up the rear end, rotate a tire by hand and have your buddy count the
revolutions of the driveshaft. Please use jack stands.
With a Sure-Grip, ten turns of a tire with both tires off the
ground will rotate the driveshaft 32.3 times for a 3.23 axle. Without
Sure-Grip, jack so as to only have one tire off the ground and rotate the free
tire twenty times. This, too, would result in the
driveshaft rotating 32.3 times for a 3.23 axle or just 27.6 times for a 2.76
axle.
Keep calm and 300
on.
Rich Barber
Brentwood, CA (cool with trace
of precip)
My research for Macungie came up with 4 F axle ratios -
2.93; 3.15; 3.54, and 3.73 - on addition to the most common
3.31.
Noel Hastalis
Sent from my
iPhone
JY and i got into pumpkin on F convert we are doing , it has 3.18 ratio ?
I believe they were factory 3:31 ? I know earlier 300's had many optional
ratios , but not sure of F ?
We are awaiting data plate decode from Gil
, have not heard from him in a while . Hope all is ok with him down there ?
In the interim -- Anyone know about axle ratio options ? 3:18 seems too
close to 3:31 to be worth doing ? A replacement ?
Also sure grip was
stand alone option ?
Sent from my
iPhone
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