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: 4,740 Florida
BDW
master
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master
Joined: May 2005
Posts: 4,740
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
Joined: Sep 2007
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: 36,041 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
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: 36,041 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
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: 4,454 Morristown Tn.
71birdJ68
master
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master
Joined: Feb 2012
Posts: 4,454
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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