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Old 08-31-2010, 09:40 PM   #1
pdora
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Default Speedometer test

Today I went on a ride and took along a friend who brought his GPS device. It can provide very accurate speed readings.

It turns out I have been driving five MPH faster than I thought.

I really enjoy using high technology in conjunction with an 80-year-old car.
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Old 09-01-2010, 08:19 AM   #2
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Default Re: Speedometer test

Just shows to go ya. You just can't trust those GPS navigators.
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Old 09-01-2010, 04:00 PM   #3
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Default Re: Speedometer test

If the speedometer is a Stewart-Warner, there is a way to adjust the accuracy of the display. Any speedometer repair shop can do that for you if you feel a need to have better accuracy. That is assuming your instrument has not deteriorated to where the adjustment range no longer is enough, AND the inaccuracy is not a result of the profile of the tires or a mismatch between the rear axle and speedometer drive gears.

Years ago, I found a S-W test machine in an Antique Shop and make that adjustment for (free) for members in our club. I set the speedometers to be accurate at 45 MPH and frequently find that at higher and / or lower speeds the readout is off by some number of miles per hour. Gar Williams
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Old 09-02-2010, 12:05 AM   #4
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Default Re: Speedometer test

Since almost all my driving is in the 40 - 45 MPH range, I will just mentally add 5 to the reading. Later on I will probably add a digital unit along with tach, temps (water, oil) etc.

Ultimately, I hope to install a computer to monitor and record all vital signs dynamically. It would be nice to include driver's point-of-view video as well.
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Old 09-02-2010, 09:53 AM   #5
marc hildebrant
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Default Re: Speedometer test

I too used a GPS to check my speed. I found that the speedometer was SLOW by 5 mph.

The car has 19 inch tires (1931) and I wonder why it's wrong ?

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Old 09-02-2010, 10:07 AM   #6
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Default Re: Speedometer test

Could be some combination of your rear end gear ratio and the speedometer drive gear teeth. I don't remember without a catalog in front of me but there is a proper drive gear in the speedometer housing for the gears you are running in the rear end. A 3:54 with the wrong spedo gear will affect the speedometer about 7 MPH. Could also be the affects of an 80 year old speedometer if it has never been rebuilt or checked.
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Old 09-02-2010, 10:21 AM   #7
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Default Re: Speedometer test

I have two GPS navigators that I checked the speedo with. One is right on the money; the other two mph off.

Still trying to figure that one out.
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Old 09-02-2010, 10:41 AM   #8
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Default Re: Speedometer test

OK got a laugh out of yesterday's post about GPS vs speedometer... I can tell you form the GPS side (that's the type of work I do for L-3), that it depends on several factors how pin point the speed is. While you can get speed from a $100 device tracking three or four SVs (Space Vehicles, not all are sat or even space based anymore) and then have another one (the one on my boat tracks up to 50) that will give you dead on.

From my exper. normally the speedo is off, usually says faster. and as stated earlier, there are ways to calibrate them but for only one speed. The farther you are from that speed the more off you are.

GPS takes an angle from each signal (usually once or 10 times a second) then does a position based on that angle and time in nanoseconds. If you are tracking 10 SVs then the current position is a avg of 10; the speed is time/distance from last position to current position. Most of the smaller cheaper GPSs are good to around 13 meters and within 8 MPH..

Hope this helps, :-)
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Old 09-02-2010, 10:56 AM   #9
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Default Re: Speedometer test

Big D,

Thanks for the info.

I checked the speedometer from 20 to 45 mph and the speed indicated was always slow by 5 mph. I did this on two separate days. While I did the test, I had about 5 signals being received.

For a speedometer that has a "good" spring, the error is constant over the range as it's just a use of Hookes' Law.

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Old 09-02-2010, 01:25 PM   #10
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Default Re: Speedometer test

Yes, that helped a lot, Big D.

I will now calibrate my GPS devices, using my accurate Model A speedometer.
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Old 08-28-2019, 08:47 AM   #11
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Default Re: Speedometer test

Hi, fairly new to my '29 Model A. The Speedo doesn't work. Watched Jack Bahm's video. The gears and spindls turn and the hairspring is ok but, the mph wheel doesn't turn when actuated with a drill motor.
Could it be the magnet?
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Old 08-28-2019, 09:00 AM   #12
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Default Re: Speedometer test

different for every jurisdiction. but there is a saying around me 8 your fine 9 your mine. they give ya 10 but I think they like the rhyme. A lot of old speedos are off and I have heard it said that calibration wasn't that big of a deal until the 60s or 70s.
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Old 08-28-2019, 09:10 AM   #13
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Default Re: Speedometer test

The speed calibration can be adjusted to be fairly accurate ( +-2 mph from 20 to 60). I have found that they are not very accurate at low or high speeds, so I calibrate to be in spec 20-60 mph. This requires an accurate reference speedometer drive with variable speed to make sure that the unit is tracking properly up and down. If the speedometer can't get up to speed with the magnet adjusted to the max, it will need to be recharged, then calibrated. This part of the restoration took a long time to learn how to do, as you are balancing two forces to get a linear reading. Any speedometer can be set to read accurately at any given speed easily, the trick is in getting it to read right over a wide range of speeds.
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Old 08-28-2019, 10:07 AM   #14
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Default Re: Speedometer test

Mark,
Thanks for a rapid reply. I can spin it fast with a drill and the magnet turns. But, when reassembled the mph dial stays at 0. Next stop is the speedo shop near me.
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Old 08-28-2019, 11:16 AM   #15
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Default Re: Speedometer test

Quote:
Originally Posted by hoarsewhisperer View Post
just shows to go ya. You just can't trust those gps navigators. :d




lol !:d
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Old 08-28-2019, 11:21 AM   #16
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Default Re: Speedometer test

Quote:
Originally Posted by pdora View Post
Today I went on a ride and took along a friend who brought his GPS device. It can provide very accurate speed readings.

It turns out I have been driving five MPH faster than I thought.

I really enjoy using high technology in conjunction with an 80-year-old car.



My speedo is off also. I learned how fast I was going years ago by other vehicles and comparing that to which thump, clank or rattle I was listening to a the time. Now the speedo is working, it didn't seem right so I checked with the GPS. Showing 40 the GPS shows 44 and at 50 the GPS shows 57. I figure as long as I know, its all good.
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Old 08-28-2019, 03:09 PM   #17
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Default Re: Speedometer test

My speedo moves from left to right a bunch of numbers so I have fun always as I have to do an "Average" and estimated the speed.

My control is one of those electronics speed detector you see in streets. There is one on my way to the supermarket. When i pass by the speed detector is always around 30-32 mph. I look my speedo and is moving left to right between 30 to 35 mph. Then i know i am in that speed.

In my case, I don't see the need of having a so perfect speedometer. When driving my A one of the last things I do is looking at the speed. In the curved and stepped streets of my MA town I really have to be concentrated in driving the car at 30 mph.... LOL .

Last edited by Forddan; 08-28-2019 at 03:17 PM.
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Old 08-28-2019, 03:53 PM   #18
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Default Re: Speedometer test

Quote:
Originally Posted by HoarseWhisperer View Post
I have two GPS navigators that I checked the speedo with. One is right on the money; the other two mph off.

Still trying to figure that one out.
In that case how do you know which one is right?
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Old 08-28-2019, 04:05 PM   #19
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Default Re: Speedometer test

Because I have an F150 gearbox my speedo is reading 75mph when I'm doing 62mph, so I use a Sat Nav (GPS) as a speedo.
One day in something of a hurry and having forgotten to attach the GPS we were cruising at what we thought was 62mph, the speed limit, (75 showing on the speedo) only to discover later that the Model A speedo won't go past 75 and realised that we must have been doing something more than 62mph for much of the trip (The car will cruise at 110kph which is around 68mph)
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Old 08-28-2019, 04:23 PM   #20
30 Closed Cab PU
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Default Re: Speedometer test

Quote:
Originally Posted by Forddan View Post
My speedo moves from left to right a bunch of numbers so I have fun always as I have to do an "Average" and estimated the speed.

My control is one of those electronics speed detector you see in streets. There is one on my way to the supermarket. When i pass by the speed detector is always around 30-32 mph. I look my speedo and is moving left to right between 30 to 35 mph. Then i know i am in that speed.

In my case, I don't see the need of having a so perfect speedometer. When driving my A one of the last things I do is looking at the speed. In the curved and stepped streets of my MA town I really have to be concentrated in driving the car at 30 mph.... LOL .

You may want to try more than one of those by the side of the road speed signs/indicators. I had the differential gearing and turtle gear changed. Tried on one of the road signs and it told me my speedometer was 4 mph fast fast at 30 mph (at 30 on my speedometer sign told me I was at 34 mph). The other day I went past a different speed sign, and my speed speedometer matched the sign.
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