Speedometer gear
#2231958
01/10/17 11:40 PM
01/10/17 11:40 PM
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Joined: Nov 2003
Posts: 549 WV
JohnH
OP
mopar
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OP
mopar
Joined: Nov 2003
Posts: 549
WV
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After completing my B Body,, total restro,, My speedometer is reading about 10 mph slow.. I pretty sure I've got 355 gears,,, 15'' tires,, about what gear do I need to put in the transmission,,, Speedometer was completely rebuild.. Thank you for any help..
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Re: Speedometer gear
[Re: JohnH]
#2232032
01/11/17 01:01 AM
01/11/17 01:01 AM
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Joined: Feb 2010
Posts: 18,493 Granite Bay CA
Kern Dog
Striving for excellence
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Striving for excellence
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
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After completing my B Body,, total restro,, My speedometer is reading about 10 mph slow.. I pretty sure I've got 355 gears,,, 15'' tires,, about what gear do I need to put in the transmission,,, Speedometer was completely rebuild.. Thank you for any help.. 10 mph slow at what speed? 40 ? 60 ? 100? Errors in accuracy are directly proportional to speed as they are the result of an incorrect ratio. The link Stumpy posted will help. Telling us 15" tires isn't enough, the diameter of the tire is the key. A super thin sidewall 195-50-15 tire will surely be shorter than a 235-75-15. A speedometer that reads too slow needs a speedo-pinion gear that is smaller. This will spin faster. The opposite is also true. In many cases, a single tooth difference can be worth 2-3 mph of change. I have a variety of used speedometer gears that I've used to correct errors. They are easily found on ebay for under $25.
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Re: Speedometer gear
[Re: JohnH]
#2232055
01/11/17 01:32 AM
01/11/17 01:32 AM
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Joined: May 2005
Posts: 4,701 Florida
BDW
master
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master
Joined: May 2005
Posts: 4,701
Florida
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Re: Speedometer gear
[Re: JohnH]
#2232065
01/11/17 01:48 AM
01/11/17 01:48 AM
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Joined: Sep 2007
Posts: 14,889 up yours
Supercuda
About to go away
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About to go away
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Posts: 14,889
up yours
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Depends on what year B body as well.
there is no one chart that covers all years.
Ball and trunnion transmissions will use a different gear than later transmissions, so a 64 B will NOT use the same gear as a 68 B for example.
Then the even later transmissions with the plastic pinion adapter will use a third type of gear, but I forget when that went into effect.
None of the three different gear styles interchange, though you can swap the second and third style as an assembly with the adapter and speedo cable.
They say there are no such thing as a stupid question. They say there is always the exception that proves the rule. Don't be the exception.
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Re: Speedometer gear
[Re: JohnH]
#2232075
01/11/17 02:20 AM
01/11/17 02:20 AM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
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I would measure rollout in a parking lot, the distance starting with a rear valve stem at 6 O'clock then rolling it straight forward till the stem is back at 6 O'clock (~8ft) then divide Pi into that number for the exact diameter. With that number & your gear ratio then pick the correct pinion teeth number from the chart CNXT posted. this will get your ODO correct & if the speedo magnetic gismo is working correct, the speedo will also be dead on (you did say it was rebuilt). what you could do right now is either use GPS if you have it or go 5 mile markers on the interstate & see if the ODO has the same error % that the speedo does & if so then with the correct tooth count swapped in the trans you should be good.
live every 24 hour block of time like it's your last day on earth
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Re: Speedometer gear
[Re: JohnH]
#2232229
01/11/17 12:44 PM
01/11/17 12:44 PM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
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Up to 12 or more difference at higher speeds. the odo or the speedo? Frankenduster, Kunkel put me on to rollout as opposed to just measureing from the underside of a piece of stock on top of the tire down to the pavement for dia.
live every 24 hour block of time like it's your last day on earth
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Re: Speedometer gear
[Re: stumpy]
#2232861
01/12/17 03:07 PM
01/12/17 03:07 PM
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Joined: Feb 2012
Posts: 4,445 Morristown Tn.
71birdJ68
master
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Joined: Feb 2012
Posts: 4,445
Morristown Tn.
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I asked several times if he tested it with highway signs. That will get to the root of the problem. Either the speedo is wrong, or the trans gear is. Like pulling hens teeth.
Last edited by 71birdJ68; 01/12/17 03:09 PM.
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Re: Speedometer gear
[Re: stumpy]
#2233682
01/13/17 07:47 PM
01/13/17 07:47 PM
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Joined: Jan 2003
Posts: 25,720 Rio Linda, CA
John_Kunkel
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 25,720
Rio Linda, CA
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In my state the painted stripes next to the mile markers are dead accurate...the airborne cops use the stripes to issue speeding tickets so they have to be accurate.
The INTERNET, the MISinformation superhighway
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Re: Speedometer gear
[Re: stumpy]
#2233835
01/14/17 12:03 AM
01/14/17 12:03 AM
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Joined: Jul 2013
Posts: 1,272 Northern Calyfornua
Sxrxrnr
pro stock
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pro stock
Joined: Jul 2013
Posts: 1,272
Northern Calyfornua
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For the price of a tank of fuel there are millions of smart phones for sale. Obama even gives them away.
Many highways in Calyfornua no longer have mile markers.
Before smart phones, I too used mile markers and a stop watch, that cost me more than a used smart phone costs today. The smart phone has proven more convenient for me and accurate and I ain't required to find no highway to check my speedo or odometer.
Could help OP with his issue.
I recently went thru a speedo accuracy issue with wife's 55 T Bird in which I had swapped in an AOD 4 speed auto transmission. I not only had to sort out mph speedo, but too an odometer accuracy problem. Could have done it with mile markers and a stop watch,,,,but would have wasted a few additional hours doing so over using this app.
60 mph, how long to go 1 mile=
120 mph, how long to go 1 mile=
90 mph, how long to go 1 mile=
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Re: Speedometer gear
[Re: JohnH]
#2233900
01/14/17 04:16 AM
01/14/17 04:16 AM
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Joined: Jul 2013
Posts: 1,272 Northern Calyfornua
Sxrxrnr
pro stock
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pro stock
Joined: Jul 2013
Posts: 1,272
Northern Calyfornua
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As an ex--fireman for the Chicago, Burlington and Quincy Railroad on the head locomotives of very high speed passenger trains such as the California Zephyr as a young man, trains sometimes running over 100 mph(speed limit 87 mph) we were always aware of our speeds.
Using either our RR certified pocket watches(or just introduced in the early 60's, Bulova Acutron wristwatches), we(Engineer or Fireman) were always train speed aware, and often would time our trains using mileposts and watches to check out the accuracy of the diesel locomotive speedometers, as accuracy was paramount.
Using this primitive technology, we would be close enough for government work,,,however if eyeballing your watch or the mile post you always were subject to a 'fudge' factor of several mph being common.
Using modern technology utilizing GPS either via a smart phone app or a Garmin type navigation device you are dead nuts on,,,,and far simpler. Calculate the answers to my 3 questions on mph and get back to me for further information, most who attempt will not have the correct answers.
It is impossible to accurately determine looking at an odometer that when the 10th mile indicator is exactly the same as it was when you started you measurement, however 5 or 10 mile measurement average will dial it in more accurately.
My point being that a GPS device is mandatory if you wish indisputable accuracy, if government work standards are sufficient and you have the time,,,mileposts and wristwatches will certainly do the job.
As for me I would prefer to have purchased and installed the correct speedometer gear on the first attempt.
The reason for suggesting to OP that a GPS device may help him in his quest.
The purpose of this forum is to help our fellow members in offering up opinions,,,,as to the most expeditious solution to problem that is presented
When I proposed a smart phone app to perhaps help him out, never occurred to me thst one should first ax if he had access to a smart phone.
Tough Crowd!
Last edited by Sxrxrnr; 01/14/17 02:39 PM.
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Re: Speedometer gear
[Re: Sxrxrnr]
#2233957
01/14/17 12:15 PM
01/14/17 12:15 PM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
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My point being that a GPS device is mandatory if you wish indisputable accuracy
The purpose of this forum is to help our fellow members in offering up opinions,,,,as to the most expeditious solution to problem that is presented
When I proposed a smart phone app to perhaps help him out, never occurred to me thst one should first ax if he had access to a smart phone.
Tough Crowd! We're the best. yes the mile markers are as accurate as an non electronic system can be but agreed I would borrow a smart phone or something similar. I catch Hell for this. I unhook the wire to disable the ringer at night.
live every 24 hour block of time like it's your last day on earth
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Re: Speedometer gear
[Re: JohnH]
#2236845
01/18/17 05:51 PM
01/18/17 05:51 PM
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Joined: Mar 2006
Posts: 74 Stevensville, ON
Frank Raso
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member
Joined: Mar 2006
Posts: 74
Stevensville, ON
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You can calculate exactly what speedometer pinion gear you need if you know the tire size and axle gears. I went through this exercise when I upgraded my 65 Barracuda to LBP wheels and have a write-up about it. Tire UpgradesWhat size tires do you have on the back?
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