Incorrect speed?

  1. WranglerMan

    WranglerMan Well-Known Member

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    That would be an interesting test, my thought was my speedo was off even before I went to larger tires
     
  2. Shots

    Shots Well-Known Member

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    Funny, because I was thinking of checking it against the stock tires to test the accuracy of the app. I suppose it could also test accuracy of the speedometer if you know the app is accurate. Good point.
     
  3. kkuntz01

    kkuntz01 Well-Known Member

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    As others have stated, your Speedo is off because you’re calibrated to the incorrect tire size. Put a level on top of the tire, center the bubble and then measure from the ground to the level. Set you tire size to the measurement, or as close as you can.

    I did this and had my 37’s within 1mph of the Jeep, external Garmin GPS, iPhone app and Escort radar detector.
     
  4. Rhinebeck01

    Rhinebeck01 Well-Known Member

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    Yes, I did check speedo for accuracy. My 2dr. JL with stock tires has speedo displaying correct MPH.
     
  5. Shots

    Shots Well-Known Member

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    I would assume the speedometer is close (within a 1-2 mph), and likely even right on when equipped with stock tires. I was thinking you could use that relative known, to test the app's accuracy prior to using it to gauge your new tires.
    The accuracy of the app is obviously going to be dependent on the accuracy of the phone's gps. If you know the phone/app report the same speed as the speedometer with the stock configuration, you could then assume the reading it shows with larger tires is accurate. You can then use the tuner to adjust the speedometer to show the same speed as the app.
    Sorry if I confused anyone. I was wondering about the accuracy of the app (or more specifically each users phone), not the speedometer. Essentially using stock as a gauge to test the app before trusting it to show a speed of a larger tire.
     
  6. Rhinebeck01

    Rhinebeck01 Well-Known Member

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    @Shots

    I'm thinking you are implying that an app that tenders you MPH is or could be reporting incorrectly. I can tell you that for a few years now, I have been using such apps to verify MPH accuracy in regard to different forms of transportation. I have never been given an inaccurate MPH reading by an app. (This is also with me using various android phones)
     
  7. Shots

    Shots Well-Known Member

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    Yes, that's what I was wondering. I assume they're pretty close now, but was curious if anyone actually tested it.
     
  8. Rhinebeck01

    Rhinebeck01 Well-Known Member

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    Obviously, if there is concern, try different smart phones and different apps but again, over the last few years, I have never had a MPH reported incorrectly.

    DO, when programming tire size, check for accuracy at varying speeds. Be sure you are testing on a highway that has little to no tree cover, no electrical lines over head, etc. that may cause the smart phone to hiccup lets say.

    It is also go to recheck speedo accuracy periodically....
     
  9. Arterius2

    Arterius2 Well-Known Member

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    Do remember that tires when under load gets squished slightly and is actually measuring less diameter than sheet specs. So your 35” tires with your vehicle on top probably measures only 34ish or something.

    I checked that my stock Sahara tires of 32” is actually set to 31.25” when I return pcm to factory stock setting
     
  10. cball508

    cball508 Active Member

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    Luckily I had the ability to test my 315s with a radar gun to dial it in. If it hasn't been said you need to get the true height as it sits on the vehicle, not the spec size. 315s are 34.5" tires but after measuring them using a level and tape measure they sat at 33.5 with approx 35 psi. I now have the speed with in 1 mph of the actual speed. I think the other great thing I gained was the vehicles mpg on the cluster. On my last 3 full ups I've only been 0.1 off from the est mpg. Not bad!
     
  11. Swanny297

    Swanny297 Well-Known Member

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    Most apps use GPS to calculate speed, there will always be a margin for error, just like when you use google maps, Waze, etc.

    No doubt they are accurate but all have some degree of error, between the programmer, app, and speedometer.
     
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