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Speedometer Calibration

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2.8K views 4 replies 4 participants last post by  77 cruiser  
N
#1 ·
Hey Team Chevelle - I've finally got to the point wear I've decided to get the speedometer in my 70 working. It started life as a malibu and has the full sweep speedo.

I’ve been reading the section on speedometer gears. Great write up.
I’ve followed the calculations, as well as, used TCI’s web site as a follow on. I’m still 10MPH too fast.

I’m running a 26.6 diameter tire, 3.55 gear, and a TH350 (1:1 final). I initially used the driven gear of 18 (18.82) as a starting point with a 7 tooth drive gear and it was way off. I kept adding teeth to slow it back down to where I am now. The current speedometer gears are 7 and 26…and going from the tach I’m still 10mph too fast. (2900rpm = 65mph)

Other than going to a speedo ratio adapter, could my issue be the 36 yr old speedometer or cable at this point? Just did this setup in a similar car with correct results. Thoughts??? anyone else experience this problem before i ginsu the dash for an autometer speedo \ tach setup?
 
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J
#2 ·
I prefer to just go out on the interstate with a stopwatch (or kitchen timer) and drive at 60 . I measure the time between mile markers and write it down. Do one mile and measure the time for 2 miles. It takes 60 seconds to do a mile at 60mph. Take several measurements , both going out and coming home so you have a sense of repeatable data. Then do the math. If your speedometer is indicating 6% low for example, then you need 6% fewer teeth on the gear that drives the cable. If you had a 17 tooth gear then you would want to go down by 6% to a 16 tooth gear. Nothing so much fun as a road test.
 
N
#3 ·
Thanks for the info - will give it a try!!! Since I'm supposed to have a 18 tooth gear and I'm at the end of the ability to add teeth (26 driven gear and still too fast) could it be the speedo???? Thx.
 
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E
#4 ·
Time to change the drive gear on the output shaft. Fewer teeth on the drive will turn the speedo driven gear slower, more teeth on the drive gear will turn the speedo gear faster. One tooth on the drive gear will make a lot of difference. Much more than than the driven gear. Good luck
Mike
 
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