adjusting speedometer for larger tires
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Junior User
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adjusting speedometer for larger tires
Looking to go from stock to 275/65/20 (34.1" and 7.9% increase over stock diameter). Years ago we used to change out speedo gears? on side of transmission to re-align speedo. How do we make this adjustment with these trucks? or does everyone just leave it alone and know they are doing about 5 mph more with larger tires when their speedo reads 60mph?
Senior User
Joined: Apr 2013
Posts: 121
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From: Kennewick, WA
Since the speedo pickup is located in rear differential, the dash reading is through the trucks computer. But, you already knew this, right??!!
A dealer can usually adjust through the computer connec up to a certain amount. You could also buy a "tuner" as they usually have a way of adjusting for the change. OR, as you already mentioned, set a base for what your reading is vs true speed.
A dealer can usually adjust through the computer connec up to a certain amount. You could also buy a "tuner" as they usually have a way of adjusting for the change. OR, as you already mentioned, set a base for what your reading is vs true speed.
Mountain Pass
Joined: May 2016
Posts: 245
Likes: 57
Forscan is free (the last time I checked) and can adjust your speedometer. I believe I used this link as guidance when I adjusted mine: https://forscan.org/forum/viewtopic.php?t=6346.
Good luck!
Good luck!
Fleet Owner
Joined: Jan 2004
Posts: 26,007
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From: Brentwood,CA
The free version of forscan will do it. The paid forescan lite will not. Unless you mean paying for the extended license? You can get a two month free trial.
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Mountain Pass
Joined: Aug 2019
Posts: 167
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From: Miami, FL
I have 285/65R20 and used FORScan to adjust my speedometer. My tires are 34.4 in diameter. I set the revs/mile constant to 620 and it is spot on vs.GPS speed. You may need to tweak the revs/mile a bit but I start there and adjust up or down.
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Mountain Pass
Joined: Jun 2016
Posts: 122
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From: Ohio
295/65/R20
Has anyone used the Forscan to adjust speedometer to 295/65R20's on a 2017?
It's been a long time since I set my other variables with Forscan. I could use some help again.
I was surprised my dealer couldn't change it. They suggested:
It's been a long time since I set my other variables with Forscan. I could use some help again.
I was surprised my dealer couldn't change it. They suggested:
FORD INLINE SPEEDOMETER CALIBRATOR (17-20 F-250 / F-350 / F-450)
Would this be easier than relearning Forscan?Last edited by Capt. Ted; May 14, 2020 at 02:02 PM. Reason: added calibrator
Postmaster
Joined: Sep 2016
Posts: 3,941
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From: Texas
Has anyone used the Forscan to adjust speedometer to 295/65R20's on a 2017?
It's been a long time since I set my other variables with Forscan. I could use some help again.
I was surprised my dealer couldn't change it. They suggested:
It's been a long time since I set my other variables with Forscan. I could use some help again.
I was surprised my dealer couldn't change it. They suggested:
FORD INLINE SPEEDOMETER CALIBRATOR (17-20 F-250 / F-350 / F-450)
Would this be easier than relearning Forscan?
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