Previous Thread
Next Thread
Print Thread
Page 1 of 2 1 2
Speedometer gear #2231958
01/10/17 11:40 PM
01/10/17 11:40 PM
Joined: Nov 2003
Posts: 549
WV
J
JohnH Offline OP
mopar
JohnH  Offline OP
mopar
J

Joined: Nov 2003
Posts: 549
WV
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..

Re: Speedometer gear [Re: JohnH] #2231973
01/10/17 11:54 PM
01/10/17 11:54 PM
Joined: Jan 2003
Posts: 32,882
Grand Prairie,Texas
stumpy Offline
I Win
stumpy  Offline
I Win

Joined: Jan 2003
Posts: 32,882
Grand Prairie,Texas
http://www.moparts.org/Tech/tech/pages/speedo.html. You need to know the actual height of your tires.

Re: Speedometer gear [Re: JohnH] #2232008
01/11/17 12:32 AM
01/11/17 12:32 AM
Joined: Feb 2012
Posts: 4,443
Morristown Tn.
7
71birdJ68 Offline
master
71birdJ68  Offline
master
7

Joined: Feb 2012
Posts: 4,443
Morristown Tn.
Does the odometer count of the miles with road markers correctly?

Re: Speedometer gear [Re: JohnH] #2232032
01/11/17 01:01 AM
01/11/17 01:01 AM
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
Kern Dog Offline
Striving for excellence
Kern Dog  Offline
Striving for excellence

Joined: Feb 2010
Posts: 18,493
Granite Bay CA
Originally Posted By JohnH
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.

Re: Speedometer gear [Re: JohnH] #2232055
01/11/17 01:32 AM
01/11/17 01:32 AM
Joined: May 2005
Posts: 4,679
Florida
BDW Online content
master
BDW  Online Content
master

Joined: May 2005
Posts: 4,679
Florida

Re: Speedometer gear [Re: JohnH] #2232065
01/11/17 01:48 AM
01/11/17 01:48 AM
Joined: Sep 2007
Posts: 14,889
up yours
Supercuda Offline
About to go away
Supercuda  Offline
About to go away

Joined: Sep 2007
Posts: 14,889
up yours
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.
Re: Speedometer gear [Re: JohnH] #2232075
01/11/17 02:20 AM
01/11/17 02:20 AM
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
R
RapidRobert Offline
Circle Track
RapidRobert  Offline
Circle Track
R

Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
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
Re: Speedometer gear [Re: JohnH] #2232136
01/11/17 06:09 AM
01/11/17 06:09 AM
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
Kern Dog Offline
Striving for excellence
Kern Dog  Offline
Striving for excellence

Joined: Feb 2010
Posts: 18,493
Granite Bay CA
I've never measured "roll-out" but I have heard the word. Thanks, Robert for that easy explanation! thumbs

Re: Speedometer gear [Re: JohnH] #2232138
01/11/17 06:16 AM
01/11/17 06:16 AM
Joined: Jul 2013
Posts: 1,272
Northern Calyfornua
S
Sxrxrnr Offline
pro stock
Sxrxrnr  Offline
pro stock
S

Joined: Jul 2013
Posts: 1,272
Northern Calyfornua
I have found this smart phone app an invaluable tool.

IMG_2385.PNG
Re: Speedometer gear [Re: JohnH] #2232188
01/11/17 11:14 AM
01/11/17 11:14 AM
Joined: Feb 2012
Posts: 4,443
Morristown Tn.
7
71birdJ68 Offline
master
71birdJ68  Offline
master
7

Joined: Feb 2012
Posts: 4,443
Morristown Tn.
Hello John, you here? What does the odometer do?

Re: Speedometer gear [Re: 71birdJ68] #2232205
01/11/17 11:53 AM
01/11/17 11:53 AM
Joined: Nov 2003
Posts: 549
WV
J
JohnH Offline OP
mopar
JohnH  Offline OP
mopar
J

Joined: Nov 2003
Posts: 549
WV
Using GPS to check my speed,, it shows 10 MPH slow according to GPS,, Up to 12 or more difference at higher speeds.. Thanks

Re: Speedometer gear [Re: JohnH] #2232229
01/11/17 12:44 PM
01/11/17 12:44 PM
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
R
RapidRobert Offline
Circle Track
RapidRobert  Offline
Circle Track
R

Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
Quote:
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
Re: Speedometer gear [Re: JohnH] #2232332
01/11/17 04:19 PM
01/11/17 04:19 PM
Joined: Feb 2012
Posts: 4,443
Morristown Tn.
7
71birdJ68 Offline
master
71birdJ68  Offline
master
7

Joined: Feb 2012
Posts: 4,443
Morristown Tn.
I said odometer, not speedometer. They work in completely different ways.

Re: Speedometer gear [Re: 71birdJ68] #2232515
01/11/17 10:01 PM
01/11/17 10:01 PM
Joined: Nov 2003
Posts: 549
WV
J
JohnH Offline OP
mopar
JohnH  Offline OP
mopar
J

Joined: Nov 2003
Posts: 549
WV
Looks like the best thing I can do is see what gear I have in the transmission first,, when I do I'll repost for your option,, I'll get back to you when I check what I have now.. and the tire diameter,, The info I got here helps a lot,,, By the way the millage works great.. Thanks

Re: Speedometer gear [Re: JohnH] #2232577
01/12/17 12:02 AM
01/12/17 12:02 AM
Joined: Feb 2012
Posts: 4,443
Morristown Tn.
7
71birdJ68 Offline
master
71birdJ68  Offline
master
7

Joined: Feb 2012
Posts: 4,443
Morristown Tn.
Does that mean the odometer is counting the miles correctly?

Re: Speedometer gear [Re: 71birdJ68] #2232715
01/12/17 11:17 AM
01/12/17 11:17 AM
Joined: Nov 2003
Posts: 549
WV
J
JohnH Offline OP
mopar
JohnH  Offline OP
mopar
J

Joined: Nov 2003
Posts: 549
WV
Answer to 71 Bird,,, yes it works good... Thanks

Re: Speedometer gear [Re: JohnH] #2232764
01/12/17 12:54 PM
01/12/17 12:54 PM
Joined: Feb 2012
Posts: 4,443
Morristown Tn.
7
71birdJ68 Offline
master
71birdJ68  Offline
master
7

Joined: Feb 2012
Posts: 4,443
Morristown Tn.
Ok, finally. Then if the odometer works as it should, that means you have the correct gear in your trans, and the speedo needs worked on. Who redid the speedometer.? The odometer is purely mechanical with gears, and the speedometer is magnetic. Not many people have the equipment to mag and demagnetize the speedometer to make it right. You can either get it repaired, or keep playing with the trans gears until it reads right, but then the odometer will either be adding miles up that you haven't driven, or not registering enough miles. My car is the same way.

Re: Speedometer gear [Re: JohnH] #2232829
01/12/17 02:29 PM
01/12/17 02:29 PM
Joined: Jan 2003
Posts: 32,882
Grand Prairie,Texas
stumpy Offline
I Win
stumpy  Offline
I Win

Joined: Jan 2003
Posts: 32,882
Grand Prairie,Texas
He says it works but not that he tested it using the hiway mile markers. So it may be off as badly as the speedo.

Re: Speedometer gear [Re: stumpy] #2232861
01/12/17 03:07 PM
01/12/17 03:07 PM
Joined: Feb 2012
Posts: 4,443
Morristown Tn.
7
71birdJ68 Offline
master
71birdJ68  Offline
master
7

Joined: Feb 2012
Posts: 4,443
Morristown Tn.
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.
Re: Speedometer gear [Re: 71birdJ68] #2233477
01/13/17 01:45 PM
01/13/17 01:45 PM
Joined: Nov 2003
Posts: 549
WV
J
JohnH Offline OP
mopar
JohnH  Offline OP
mopar
J

Joined: Nov 2003
Posts: 549
WV
I didn't check it on highway signs,,, I will when the weather breaks,,, dry roads, no salt..

Page 1 of 2 1 2






Powered by UBB.threads™ PHP Forum Software 7.7.1