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Thread: HOWTO: Ford Speedo Correction

  1. #1
    HPT Employee Eric@HPTuners's Avatar
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    Lightbulb HOWTO: Ford Speedo Correction

    A few people have asked me how to correct the reading of their speedometer due to tire size changes.

    To make a correction, the very first thing that has to be done is setting the "Use VID" parameter to disabled. If this is not done, the changes you make to the "Tire Revs per Distance" parameter will not make a difference.



    If you have aftermarket tires and are lucky then you may be able to find correct value to use for Tire Revs/Mile off of the tire manufacturer's website.

    If you are unable to find the information then you can use a GPS to measure your speed and compare to the speed the PCM is reporting to come up with a correction factor.

    Example: Your speedometer reports 55 MPH while the GPS reports 60 MPH. You would take 55 divided by 60 and get a correction factor of .916. You would then take your current value for "Tire Revs per Distance" and multiply it by this correction factor to come up with the new value.

    This usually is enough to get it pretty spot on, but you can repeat the process a second time to get it a little closer.

  2. #2
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    Should sticky this. . It's already moving down the list.

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    Bill@HPTuners's Avatar
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    done, I was meaning to do this the other day but got distracted and forgot about it.
    It doesn't have to be perfect, it just needs to be done in two weeks...

    A wise man once said "google it"

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    Potential Tuner REVXP800R's Avatar
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    hey guys i have searched everywhere and for whatever reason cannot figure this out....

    05 F150 FX4

    my tire size went from 275/65r18 to 325/65r18

    i found a thread stating to change the final drive gear ratio

    so i tried, i went from my stock 3.73 gears and changed it to 3.44 which would be the overall gear ratio with the new tire size. honestly im not versed enough in this to even tell if it made a difference, and also there is snow everywhere on the roads so this is also affecting things.

    anyways i am 5kmh out on my speedo, and i need to change this, and the tire and gear calculator just confuses me...

    i need some definitive numbers and exactly how and where to put them. i am not looking to tune my truck myself anymore, i just want to correct my speedo and ensure the shifting is correct.
    05 F150 Fx4 Crew
    08 Rev XP 800R

  5. #5
    Potential Tuner REVXP800R's Avatar
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    i re-read, and disabled the "use vid"

    tire revs are 597/mile which works out to be about 373.125 per km, however every time i try to edit this it automatically changes it to 372.82......

    so confused....
    05 F150 Fx4 Crew
    08 Rev XP 800R

  6. #6
    HPT Employee Eric@HPTuners's Avatar
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    Email the file to support

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    Hmm. So I opened my file on my 2013 F150 and it looks nothing like above. I have no revs/mile and only driven tire circumference. I am assuming I will just have to mod the value to correct my speedo.

  8. #8
    HPT Employee Eric@HPTuners's Avatar
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    Quote Originally Posted by Badass69 View Post
    Hmm. So I opened my file on my 2013 F150 and it looks nothing like above. I have no revs/mile and only driven tire circumference. I am assuming I will just have to mod the value to correct my speedo.
    http://www.hptuners.com/forum/showth...l=1#post365072
    Eric Brooks
    HP Tuners, LLC

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    I am needing to change gear ratios in my 2014 Mustang and have already modified the Gear Ratio but It didn't make a difference. I also have no option to disable VID. Any thoughts?
    MustangVID.png

  10. #10
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    I dont know what changed but when I opened 2.25 today and opened the speedometer, the right functions were there. I disabled VID and it worked great.

  11. #11
    Tuner JnJSpdShop's Avatar
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    Eric if you just change the gear ratio and leave the tire size the same do you still need to Disable VID?

    Western New York's Performance Headquarters

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    Quote Originally Posted by JnJSpdShop View Post
    Eric if you just change the gear ratio and leave the tire size the same do you still need to Disable VID?
    In my experience yes.

  13. #13
    HPT Employee Eric@HPTuners's Avatar
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    Quote Originally Posted by JnJSpdShop View Post
    Eric if you just change the gear ratio and leave the tire size the same do you still need to Disable VID?
    Yes, it will use whatever is in the VID block if you don't flip the switch.
    Eric Brooks
    HP Tuners, LLC

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    Having issues changing speedometer on 2015 f-150?? Tried it maxed out to zeroed out and speedometer never changed. Still about 4 mph off by GPS.
    Is this not working on the 15s yet? Currently on version 3.0

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    Senior Tuner Higgs Boson's Avatar
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    it works on my 2016 mustang.

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    Quote Originally Posted by Higgs Boson View Post
    it works on my 2016 mustang.
    Are you sure, I was under the impression HPTuners would never be able to adjust speedo as it's in the BCM, not the PCM. Need Whipple's device or Ford unit of some sorts.

  17. #17
    Senior Tuner Higgs Boson's Avatar
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    I know that's what I thought too. Maybe I will try a dramatic change to be sure.

  18. #18
    Tuner TunedByNishan's Avatar
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    Another way to figure out the Tire Rev's Per Mile is by calculating it.

    First, calculate your overall wheel diameter.

    I will work out an example:

    295/55/15

    295mm * 0.55(2) = 324.5mm ----------- the 0.55 aspect ratio is multiplied by 2 because the overall diameter consists of 2 times the sidewall height.

    Convert to Inches --> 324.5 / 25.4 = 12.77"

    12.77" + 15" wheel = 27.77" wheel diameter.

    There are 63,360 inches in 1 mile. 3.14 = pi

    63,360 / (27.77 * 3.14) = 726.1 revolutions per mile

    Enter that value into the PCM.

    Everytime I have done this, the speedo is quite dead accurate with a GPS or radar sign and any small amount of error due to differences in tire manufacturers is so small you can't even notice it on a low resolution speedometer.
    www.tunedbynishan.com
    2018 Mustang GT - 10R80

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    i have a 2016 mustang gt auto change diff from 3.15to 3.73 but no option greater than 3.55

  20. #20
    HPT Employee Eric@HPTuners's Avatar
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    This thread does not apply to 2015+ F-150 and Mustangs. The axle ratio and tire size values are stored in the body control module. To change it, use VCM Scanner to make the change.
    Eric Brooks
    HP Tuners, LLC