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Posted by Justin VanAbrahams [Email] (#32) [Profile/Gallery] (more from Justin VanAbrahams) on Thu, 31 Mar 2016 15:56:00 Share Post by Email
In Reply to: Replacing the Speedometer, Justin VanAbrahams [Profile/Gallery] , Mon, 28 Mar 2016 14:32:33
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On the way to work today I remembered I have a half-dismantled transmission in the garage, so at lunch I checked it out.

The speedometer drive gear on the 900's inner driver has 16 teeth, the gear on the speedometer pickup itself has 13 teeth. I have a LOI of c900 pickups, they're all 13 teeth. :) A 1.23:1 ratio.

I also found a 9000 speedometer gear which has more teeth (14, IIRC), but I don't have the drive gear so that's useless info. :)

2485 square pulses per kilometer = approx 3965 square pulses per mile

VSS is 4x the square pulse rate, or approx. 15860 sine pulses per mile

Indicated drill speed was 700rpm, divided by the 1.23:1 ratio of the speed drive, that approximately 569rpm at the wheel.

195/60-15 tire is approximately 76.0194" around. 569rpm * 76.0194" tire circumference is 43263" per minute, or approximately .68 miles per minute

.68 miles per minute * 15860 sine pulses at one mile per minute is approximately 10829 sine pulses per minute

10829 sine pulses per minute divided by 60 seconds per minute = 180 sine pulses per second.

I measured 189.4Hz. 189.4Hz is SO CLOSE to the expected 180Hz I think it's fair to say it's close enough. That's 4% error, well within speedometer inaccuracy and possibly explained by a different tire size (195/60-15 c900 vs. 195/65-15 or 205/55-16 9000) or possibly explained by a slight error in the speedo drive ratio, 900 vs. 9000.

I am pretty satisfied this is an accurate test with good math.

The only thing I'd enjoy discovering is what the 9000's speedo gear ratio is, just for grins.


posted by 12.195.130...

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