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Old 10-15-2018, 02:13 PM   #1
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Son in Law rear end question

My son in law was out of town and being the so called mechanic he is put a complete rear end in my truck the old one tore up he said have not looked but the ratio was a 3.92 an to begin with he put a 3.55 in it does ok but wont shift right and of course mph is off can i reprogram to fix that or should i try to fix or find a a 3.92 Thanks

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Old 10-15-2018, 03:35 PM   #2
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Is it a 4wd truck? If it is you have to change the gears out to match one or the other. You’ll blow up your transfer case. Gear changes shouldn’t effect the speedometer, the dodges use a speed sensor in the differential or in the front abs hub so the gear don’t change the speedo, only tire size changing.
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Old 10-15-2018, 04:43 PM   #3
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Like stated if 4x4 until fixed remove front driveshaft at least. You can adjust the Speedo with a tuner or a trip to the dealer if you want to keep the 3.55 ratio.
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Old 10-15-2018, 04:45 PM   #4
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Quote:
Originally Posted by charged392 View Post
Like stated if 4x4 until fixed remove front driveshaft at least. You can adjust the Speedo with a tuner or a trip to the dealer if you want to keep the 3.55 ratio.
I thought the speedo stays the same with a gear change? Mine seemed to have after the 4.56's
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Eagle Heads and Intake Manifold-
46RE-CRT Billet Manual Valvebody-Edge 3200 convertor
4.56 and Detroit TruTrac-Custom Morimoto Projector Headlights
Tuned by Charged392
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Old 10-15-2018, 04:54 PM   #5
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It may be off just a touch speed match another car and see is how I check
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Old 10-15-2018, 06:28 PM   #6
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VSS is read off of the differential tone ring and has nothing to do with gear ratio. It simple meases differential rotation. Speedo adjustment is only needed if tire height is changed and then it is adusted in the tune.
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Old 10-15-2018, 07:32 PM   #7
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My speedometer was off by 4 mph, I used GPS to determine speed of truck, then adjusted with my SuperChips CORTEX tuner. Reverified speed with GPS. I am now about 1/2 mph off, reading high, works for me.
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Old 10-16-2018, 01:34 AM   #8
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son in law

its a 2wd hemi from this its got to go to dealer its does not shift right hope he did not destroy tranny Just bought a 2nd beater truck so when his breaks he can drive that
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Old 10-16-2018, 05:13 AM   #9
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The 3:55 was often found on the base line trucks with 17" wheel from the factory. The 3:92 mostly came with the 20" wheel package and traction control.
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Old 10-16-2018, 11:03 PM   #10
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I did a gps test and followed my wife’s car tonight. My speedometer read the same across everything. I changed my gear ratios from the factory 3.92 to 4.56 with no adjustments
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Eagle Heads and Intake Manifold-
46RE-CRT Billet Manual Valvebody-Edge 3200 convertor
4.56 and Detroit TruTrac-Custom Morimoto Projector Headlights
Tuned by Charged392
Tons of Pics https://www.flickr.com/photos/[email protected]/albums
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