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
2.93: 44/15 or 41/14=2.9393 or 2.9286
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)
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.
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.
Posted by: "Rich Barber" <c300@xxxxxxx>
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