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More fun with numbers. Most mechanical-drive speedometers are calibrated to display 60 MPH (or one mile/minute) @ 1,000 RPM of the speedometer drive cable. One can use any of the following expressions of the same equation for a precise computation of the ideal number of teeth on the driven (pinion) gear in a speedometer cable drive system: # of teeth on pinion gear=((# of teeth on driving gear) X (Axle Ratio) X (Tire revolutions/mile))/1,000 =(# of teeth on driving gear) X (Axle Ratio) X ((5,280 x 12)/(Pi x Tire OD in “))/1,000 =(# of teeth on driving gear) X (Axle Ratio) X (20.168/OD) For our 8-tooth driving gears: # of teeth on pinion gear = (Axle Ratio X 161.344)/(Tire OD) The 1964 Chrysler Service Manual lists the number of teeth on the driving gear is 8 and suggests a 19-tooth pinion gear for either the 8.00 X 14 (27.58” OD) or the 8.50 x 14 (28.10” OD) tire with the 3.23 axle. Calculated values for number of teeth are 18.89 and 18.54, respectively. With driven gears having numbers of teeth in the 18-20 range, one can expect accuracies of approximately +/- 2.5%. Using the 19-tooth gear would cause the speedometer to read 58.8 MPH at an actual 60 MPH and the 18-tooth gear will cause the speedometer to read 62.0 MPH at an actual 60 MPH. For the example of a 235/75R x 15 tire with a nominal OD of 28.90” and a 2.93 axle, the ideal pinion would have 16.35 teeth. Significant variance from predicted speedometer readings may indicate the possibility of the speedometer head drifting out of calibration. From the internet: “Recalibrating a speedometer can be done by manipulating the hairspring, the permanent magnet or both. Generally, the strength of the magnetic field is the easiest variable to change. This requires a powerful electromagnet, which can be used to adjust the strength of the permanent magnet in the speedometer until the needle matches the input from the rotating drive cable.” Changing pinion gears may be an inexpensive fix for an out-of-calibration speedometer as the correction would be linear with speed. I would expect that adjusting the hairspring on the needle would produce a fixed adjustment—say 4 MPH at all speeds while recalibrating the strength of the fixed magnets might produce the needed linear adjustment—say 5% at all speeds. I recall hearing information that highway patrol cars and others had to have their speedometers calibrated periodically in order to stand up in court. I have also seen information that police and highway patrol cars come with different speedometers—perhaps more stable and more easy to calibrate. Rich Barber Brentwood, CA 1964 300K Convertible (with a grossly under-sized speedometer—anyone know why? PC in ’64? I recall that at one time the idiotic car nannies enacted a maximum speedo reading of 85. Wiki: Speedometers[edit] On September 1, 1979, in a regulation that also regulated speedometer and odometer accuracy, the National Highway Traffic Safety Administration (NHTSA) required speedometers to have special emphasis on the number 55 and a maximum speed of 85 mph (135 km/h). However, on October 22, 1981, NHTSA proposed eliminating speedometer and odometer rules because they were "unlikely to yield significant safety benefits" and "[a] highlighted '55' on a speedometer scale adds little to the information provided to the driver by a roadside speed limit sign."[37][38][39]From: Chrysler300@xxxxxxxxxxxxxxx [mailto:Chrysler300@xxxxxxxxxxxxxxx] On Behalf Of lindsey@xxxxxxxxxxxxxxx [Chrysler300] Jeff carter might have some gears. Got one from him a few years ago. Sent from my BlackBerry 10 smartphone on the Rogers network.
So that's how you know you were doing 106 mph!! From: "'Bob Jasinski' rpjasin@xxxxxxxxxxx [Chrysler300]" <Chrysler300-noreply@xxxxxxxxxxxxxxx> I appreciate the feedback. I think the best approach is for me to first identify what is in there now, and then find the next higher pinion size, 1-2 tooth higher, and try that. I have two electronic speed readout signs that I regularly cruise by to use as a reference. Bob J From: Chrysler300@xxxxxxxxxxxxxxx [mailto:Chrysler300@xxxxxxxxxxxxxxx] On Behalf Of john begian r2gthawk@xxxxxxxxx [Chrysler300] I run 225/75-15 tires, with a 3.23 ratio. The standard 18 tooth red gear had the speedo reading high, I was actually going 55 and speedo showed 62. Put in the black, 19 tooth gear and all is well, I only show 2mph higher than actual now. Good luck finding these gears, CI torqueflight specific and they are scarce. John Begian On Friday, July 17, 2015 1:49 PM, "John Holst jholst@xxxxxxxxx [Chrysler300]" <Chrysler300-noreply@xxxxxxxxxxxxxxx> wrote: As a check of the speedometer I have used a portable GPS readout (hand held). My RAM Charger is on it's third speedometer and reads about 10 mph slow at 55. I'm sure that somewhere I got the wrong cable or gear or something, but for pushing snow it works. John Holst ( i have a sign on the dash that warns people not extend arms outside the window at speeds above 120 mph)
 I'm trying to determine the proper speedometer pinion gear for my 300G. My speedo reads lower than it should and I'd like to see if I can improve its accuracy, at 40 mph I'm actually doing about 45. I'm running 235/75R15 tires and a 2.93 rear axle. Any suggestions on the proper gear? I know they come in different colors to differentiate different ratios.  Thanks!  Bob J  __._,_.___ Posted by: "Rich Barber" <c300@xxxxxxx> To send a message to this group, send an email to: Chrysler300@xxxxxxxxxxxxxxx To unsubscribe from this group, send an email to bob@xxxxxxxxxxxxx or go to https://groups.yahoo.com/neo/groups/all/manage/edit For list server instructions, go to http://www.chrysler300club.com/yahoolist/inst.htm For archives go to http://www.forwardlook.net/300-archive/search.htm#querylang __,_._,___ |
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