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Old 09-22-2013, 10:14 AM
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gearing change-speedo Q

Im going to pull my f250 axles/springs with 4.10's and put in f350 springs/axles with 3.55's, Question i have is im assuming the speedo drive is in the trans, and set for 4.10's, whats the best way to get it reset so the speedometer is correct with 3.55's?
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Old 09-22-2013, 11:43 AM
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the speedo runs off of the tone ring in the rear. you won't need to do anything
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Old 09-16-2014, 06:57 AM
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So how do you correct for diff. size tires?
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Old 09-16-2014, 07:32 AM
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that is done through the PSOM, the speedometer in the cluster.
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Old 09-16-2014, 07:33 AM
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PSOM? If I go to 3.73 running my current tire size 295/75/16 I am trying to figure out my rpm at 70 MPH
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Old 09-16-2014, 07:36 AM
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RPM = MPH * Gear Ratio * 336 / Tire Diameter
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Old 09-16-2014, 07:40 AM
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I am not too sure I fully understand.
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Old 09-16-2014, 07:49 AM
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(70mph x 3.73 (gear ratio) x 336)/33.49 (tire size in inches) = RPMs

RPMs = 2619
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Old 09-16-2014, 07:53 AM
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How do I take in account for the PSOM being off?

Right now doing 70 MPH the RPM's are 2250 or so with my 4.10
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Old 09-16-2014, 07:54 AM
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BTW that means that at 70 MPH on the speedo I am really doing 75MPH
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