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Re: how many tooth gear for 3:55 gear speedo?
[Re: CUDA8U]
#1306716
09/18/12 11:48 PM
09/18/12 11:48 PM
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Joined: Jan 2003
Posts: 12,043 State of Confusion
hp383
Just a normal tag again
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Just a normal tag again
Joined: Jan 2003
Posts: 12,043
State of Confusion
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This will depend on your tire diameter as well.
There is info in the Moparts tech archives with chart!
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Re: how many tooth gear for 3:55 gear speedo?
[Re: hp383]
#1306719
09/21/12 06:54 AM
09/21/12 06:54 AM
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Joined: Jun 2008
Posts: 10,847 Oakdale CT
gdonovan
I Live Here
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I Live Here
Joined: Jun 2008
Posts: 10,847
Oakdale CT
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Quote:
This will depend on your tire diameter as well.
There is info in the Moparts tech archives with chart!
Take the chart with a grain of salt and be ready to order another gear, I had to.
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Re: how many tooth gear for 3:55 gear speedo?
[Re: BigBlockMopar]
#1306721
12/22/12 07:43 PM
12/22/12 07:43 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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Go with the GPS. That's good that the speedo and the odo are off the same amount as that tells you the speedo magnetic gismo which is triggered by the odo is spot on. As you know just need to get the correct speedo pinion gear in the trans. If that (44 tooth) gear ain't commonly available then might need to source a speedo gear adapter which has metal gears of a different ratio in a little box that goes inline in the speedo cable to alter the ratio so you're final ratio will come in correct
live every 24 hour block of time like it's your last day on earth
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Re: how many tooth gear for 3:55 gear speedo?
[Re: BigBlockMopar]
#1306722
12/22/12 08:17 PM
12/22/12 08:17 PM
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Joined: Jan 2003
Posts: 25,821 Rio Linda, CA
John_Kunkel
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 25,821
Rio Linda, CA
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Quote:
All the formulas I've found online say my Dart with 518OD, 3.91 gears and 26" tires need a 40-tooth gear. Still, measured against GPS, the speedo is exactly 10% off, both in speed and distance. So I need to get me a 44-tooth gear-unit, which don't seem too commonly found.
With a 26.2" tire the pinion tooth count will be the same as the ratio i.e. a 4.10 axle will use a 41 tooth pinion, etc.
Your setup requires a 39 (or 40) pinion; if the GPS shows your odometer is off that much I'd suspect your measured distance is wrong or your GPS isn't accurate.
The INTERNET, the MISinformation superhighway
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Re: how many tooth gear for 3:55 gear speedo?
[Re: BigBlockMopar]
#1306724
12/22/12 10:19 PM
12/22/12 10:19 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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Step 1 would be to get the ODO reading right with the correct pinion gear in the trans. I thought GPS's were dead nuts accurate but Kunkel knows more about this than I do. According to the GPS the speedo is reading 11% fast (70 divided by 63) so you would want a speedo gear with ~11% more teeth. I would double check by measuring rollout by putting a rear tire valve stem at 6 0'clock & making a mark on the pavement at that point then rolling the car forward 1 revolution till the stem is back at 6 0'clock & with your tape measure that distance (rollout) then divide Pi (3.1416) into rollout to get actual diameter then going to the MP chart in the archives along with the rear gears/dia so see if their recommended pinion tooth # matches. When you get the ODO confirmed accurate then see if the speedo is accurate either by GPS or a roadside digital MPH sign. If the odo is accurate but the speedo ain't then the speedo head needs to come out & go to a speedo specialty shop to be recalibrated but one thing at a time.
live every 24 hour block of time like it's your last day on earth
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