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Old 09-14-2020, 04:26 PM   #1
Ordsgt
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Default speed error

New to this so bear with me. Bought a 1930 Tudor been sitting for a couple of years. Rewired complete, changed all fluids adjusted brakes put in a 6v alternator and electronic ignition. Old speedometer did not work so got a rebuilt on. Now comes the issue. Car speed showing is 38mph but my phone gps puts speed at 50mph. Everything is about 12mph off although slightly less at real slow speeds. What is the best way to correct?
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Old 09-14-2020, 04:33 PM   #2
Patrick L.
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Default Re: speed error

These speedos are notorious for being off. The problem can be the speedo, drive gears, for the rear end ratio.
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Old 09-14-2020, 04:38 PM   #3
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Default Re: speed error

You may need to rebuild your speedometer. There are members on this board that can help you with that.

Other options could be wrong gear in speedometer turtle, different gearing in the differential, non-standard tire size.

Are the tires 19" ? Is the rear end the original ratio ? What is the number of teeth on the speedometer gear (should be on the turtle)?
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Old 09-14-2020, 04:48 PM   #4
Chuck Sea/Tac
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Default Re: speed error

A friend adjusted the magnet on my Tudor several years back. It was similar to your problem. I called him up and thanked him for now being able to do 50 in 2nd gear. He had inadvertently turned the adjustment the wrong way!
You can tell if your gears are correct by timing a measured mile and calculating if it matches your odometer. The odometer is controlled by gears and the speedometer is controlled by a magnet.
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Old 09-14-2020, 05:18 PM   #5
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Default Re: speed error

Thanks for the input. I will drive a measured mile and see if odometer is correct
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Old 09-14-2020, 05:36 PM   #6
Ruth
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Default Re: speed error

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I have the same problem. According to my Garmin my speedo reads ~10mph slow at 35 mph. My odometer seems to be pretty close. After a 30 mile trip the odo and GPS differer only .3 miles. One of these days I may have the speedo cleaned and calibrated.
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Old 09-14-2020, 09:17 PM   #7
Ordsgt
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Default Re: speed error

My speedo is completely rebuilt from Brattons. I have put 51 miles on it
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Old 09-14-2020, 09:31 PM   #8
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Default Re: speed error

Quote:
Originally Posted by Ordsgt View Post
My speedo is completely rebuilt from Brattons. I have put 51 miles on it
In that case, other things affecting the indicated speed are:

Tire and wheel size
Speedometer gear at the back of the U-Joint
Rear end gear ratio

All these things are variable and work in conjunction with each other
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Old 09-15-2020, 08:17 AM   #9
Bob Bidonde
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Default Re: speed error

GPS measurement of speed has signal processing and transmission errors. RADAR is much more precise, so find a RADAR speed indicator sign you can drive by to check your speedometer. Typically RADAR speed indicators are at school zones.
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