[C1] '62 speedometer cable speed
Thread Starter
Safety Car
Joined: Dec 1999
Posts: 4,394
Likes: 1,188
From: Lexington,NC,USA
C1 of Year Finalist (track prepared) 2019
'62 speedometer cable speed
How fast should the speedometer cable be turning at an indicated 60 MPH (or any other speed) ?
Charles
Charles
Le Mans Master
Joined: Nov 2015
Posts: 7,463
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From: Really Central IL Illinois
Depends on the diameter of the drive gear and the driven gear, much like the differential gear calculations. To calculate it you need the speedometer gear diameters in the transmission as well as the differential gears and the tire diameter.
Racer
Joined: Jun 2018
Posts: 290
Likes: 117
From: ashburnham, MA
Regardless of all the factors (tire size, gear ratio, speedo gears, etc., (which happen before the spinning of the cable) there should be a cable rpm that produces a 60 mph speedometer reading on the bench. I'd bet Jim has it right.
Ray
Ray
Race Director
Joined: Feb 2007
Posts: 15,455
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From: northern california
C2 of Year Finalist (track prepared) 2019
You calculate the answer.
Assume you have a tire which makes 750 revolutions per mile and your car has a 3.70:1 differential. At 60 MPH, the driveshaft will be turning 2775 RPM.
Now assume the speedometer drive gear on your transmission output shaft has 8 teeth..... the appropriate number for a 3.70 differential. If the driven gear has 22 teeth then the speedo cable will be turning:
2775 * 8 / 22 = 1009 RPM @ 60 MPH.
Close enough to 1000 RPM for Gummint work.
Assume you have a tire which makes 750 revolutions per mile and your car has a 3.70:1 differential. At 60 MPH, the driveshaft will be turning 2775 RPM.
Now assume the speedometer drive gear on your transmission output shaft has 8 teeth..... the appropriate number for a 3.70 differential. If the driven gear has 22 teeth then the speedo cable will be turning:
2775 * 8 / 22 = 1009 RPM @ 60 MPH.
Close enough to 1000 RPM for Gummint work.
