USA shows wrong speed

ianmetz Registered Users Posts: 11
Legendary Explorer
I've just spent three months driving in seven US states, WA, OR, ID, ND, MN, MT and CA.

In each of those states the app showed my speed as 10mph greater than it actually was. I meant to take a look at whether it was correct at ANY speed but forgot.

I attach a screenshot where I was traveling at exactly 70mph, app said 80mph..

All software is up to date. The same applied to both my iPhones, a 6+ and an X.

Any ideas?



  • Niall
    Niall Posts: 11,140

    Have you checked your speedometer against one of those speed indicators at the start of most major roadworks on the interstates?

    Vehicle speedometers are notoriously inaccurate see

    Also are driving on over-sized tires?
  • ianmetz
    ianmetz Registered Users Posts: 11
    Legendary Explorer
    Hi, it was a bog standard Ford Explorer, and no, I didn't actively check against the signs you suggest, or the mile markers, but in the wide open spaces through which I drove virtually everybody is on cruise control, and I was doing the same speed as those around me.

    It happened time after time in different states with different traffic. Even through road works where "FINES DOUBLE" so everybody's extra cautious.

    At 55mph, 60mph, 65mph, 70mph and 80mph.

    Always exactly 10mph out. Definitely the TomTom!
  • Niall
    Niall Posts: 11,140

    On the various occasions I have driven in Florida with my GO 5200 I have found that most US drivers are exceeding the speed limit, yet when in Europe the same device is VERY accurate as against the various speed displays here. Be aware that, as your TomTom calculates your speed by using up to twenty satellite's time signals, whereas your vehicles speedometer rely's on the circumference of your tire to be constant and has a manufactures dependent formula to give you the displayed speed (very iffy).

    Just because the crowd is scampering towards the abyss does not mean that their speed is accurate.

    If you do not believe me try getting a state trooper to verify your speed :) (without getting a ticket of cause)
  • ianmetz
    ianmetz Registered Users Posts: 11
    Legendary Explorer
    You appear desperate to 'prove' the satnav is accurate. You are wasting my time.

    I'm sure your experience of driving abroad is exhaustive - I've only driven in 46 US states and 22 European countries..

    So unless you've got something useful to add please find somebody else to bug.

    Thank you.
  • Niall
    Niall Posts: 11,140

    Sorry that you take that view, but did you actually read the article I linked to?
    Have you compared your vehicle's speedometer with various other speed calculations?
    Was your speedometer always the same difference? (Please give the range of variance)
  • Lochfrass
    Lochfrass Posts: 18,482
    Do you use carplay?
    In German Forum exist a similar thread
    So far TomTom on iphone has been quite plausibly showing the current speed.
    Recently, this speed is significantly too high, sometimes more than 12 km / h over car speedometer (instead of credible 5-10 as before)
    The effect seems even more powerful when using CarPlay (on Ford Sync3).
    The last TomTom update is a long time ago, in time I could bring it more in a context with the last ios update.

    Does anyone have any idea why this could be, Tips to remedy?
    So - today I came again to try out:

    My iOs compass app does not show speed ...
    Now for the essence:
    As soon as my iphone is connected to the Sync3 via USB and carplay is activated, TomTom and other apps (speedometer ...) display higher speeds.
    Without carplay plausible something under the car's roof.
    So no special TomTom problem, but generally ios carplay at least at Ford Radios ...
    Anyone this ideas?

    I mean that there is a more recent thread, but unfortunately can't find it
  • ianmetz
    ianmetz Registered Users Posts: 11
    Legendary Explorer
    edited August 2019


    Thanks for the message. Interesting!

    To answer your question, I did both... early in my trip I used CarPlay, later I stopped as I thought that may have been the issue. (I also found it a somewhat annoying device.)

    The results were exactly the same (a 10mph discrepancy)... on both iPhones... each running a different iOS.

    But it's good to see I am not alone with this issue!

    Next trip to the US I'll do more tests.

    Thanks again.

  • Don2641
    Don2641 Registered Users Posts: 19
    Outstanding Wayfarer
    I am a Canadian and i go frequently to Florida. During my travel when i cross a radar display speed, the speed mark on the radar is the same i saw on my GO 620 Wifi.
  • ianmetz
    ianmetz Registered Users Posts: 11
    Legendary Explorer

    That’s great! Thank you so much!
  • Fred_CoolOut
    Fred_CoolOut Registered Users Posts: 3
    Neophyte Traveler
    I also have the same issue. As soon as I connect to CarPlay on my car (Mercedes 2017 E300) the MPH display the KPH speed. When I disconnect the iPhone off of CarPlay the MPH works normal again on the App. I’m theorizing that iOS uses the car speedometer but replaces MPH with KPH. In the app and car settings I have under MPH units.
  • Herbaldew
    Herbaldew Registered Users Posts: 35
    Master Explorer
    Same issue here as well.

    Via CarPlay: 44MPH actual speed shows as 50MPH on car screen.

    Via the iPhone only: 44MPH actual speed shows as 44MPH on phone screen.

    The higher the actual speed, the larger the discrepancy.

    First time - every time.