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Old 12-31-2011, 03:03 PM
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Speedometer Operation/Calibration 1949 Chevry Truck

I just had a 1949 C3100 pickup restored with an upgraded drive train. I installed original reconditioned gauges in my truck and I am trying to get the speedometer calibrated. Does anyone know how many revolutions the old speedometers turn at 60 MPH? I found a formula to calculate the gears needed in the transmission, but it is based on modern speedometers which turn 1000 RPM at 60 MPH. Using 1000 in the calculations, and installing the gears it recommended, my speedometer is registering 15 mph too fast. Using my GPS unit, when I was actually driving at 45 MPH, the speedometer was reading 60. If I knew how the old speedometers work, it would help out greatly. Thanks.

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Old 12-31-2011, 04:01 PM
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Standard speedometer calibration from about 1915 on was 1000 rpm at 60 mph. Everybody used this cal.
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Old 12-31-2011, 04:24 PM
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For clarification, the speedometer turns 1000 revolutions to register 1 mile.
The amount of time it takes to do it is your speed. You have to be sure that your odometer is reading correctly before you try to calibrate for speed.
Think about it, if your speedo turns 1000 revolutions in 60 seconds then you are going 60 mph.
If you calibrate your speed without having the distance traveled correct first your speedo or Odometer will never be 100 percent correct.
Most speedos have a number in small print on them. Like 940 or 960 or 1000 or 1040. That is the revolutions per mile.Most american made stuff uses 100 revolutions per mile.
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Old 12-31-2011, 06:22 PM
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Speedometer error, for mechanical, cable0drive types, is always a percentage. Your error isn't 15 MPH, it's not gonna read 16 when you're idling in first at 1 MPH.
I'm not sold on the notion that ALL were 1000 per mile. I recall reading elsewhere about other counts per mile. In this case, it may well be 750.
You could stack a pair of correction boxes, if the needed tooth counts aren't offered for the speedometer gears intended for 1000/mile.
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Old 12-31-2011, 07:24 PM
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Quote:
Originally Posted by latech
For clarification, the speedometer turns 1000 revolutions to register 1 mile.
The amount of time it takes to do it is your speed. You have to be sure that your odometer is reading correctly before you try to calibrate for speed.
Think about it, if your speedo turns 1000 revolutions in 60 seconds then you are going 60 mph.
If you calibrate your speed without having the distance traveled correct first your speedo or Odometer will never be 100 percent correct.
Most speedos have a number in small print on them. Like 940 or 960 or 1000 or 1040. That is the revolutions per mile.Most american made stuff uses 100 revolutions per mile.
Whoops. last sentence should read 1000 not 100.
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Old 12-31-2011, 08:56 PM
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It was explained to me that the magnets in the speedo get old and need to be replaced. Was your speedo calibrated when you had it reconditioned ?
I have the same problem with mine. I changed the driven gear until I got a speedo reading that was close to the GPS and also jived with the formula. However I was racking up more miles then actual.
Evidentally something in the speedo is slowing down or dragging and to compensate I speeded it up by changing driven gear. Then that increases the odometer reading. That makes sense to me and I will eventually get the speedo recalibrated.
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