Anonymous asked in Cars & TransportationMaintenance & Repairs · 6 days ago

Do new cars require calibration?

My speedometer bar isn't matching the digital speedometer. And when I set cruise control, it always goes a mile over. It thinks there's a passenger when I have my backpack or grocery or anything on the seat and alerts to put seat belt on 😑

4 Answers

  • Neil
    Lv 7
    6 days ago
    Favorite Answer

    There is a very simple weight sensor in the passenger seat, so if you set anything on the seat it will assume that is a passenger. However, as you should never have anything unrestrained on a seat in any case, this should not be a problem - either strap your backpack in so it is restrained, or move it to the luggage compartment (or passenger footwell).

    Is the "digital speedometer" part of the sat-nav system?

    If so it will not match the main speedometer, partially because it is measuring the speed differently, by plotting your position on the earth every few seconds, and comparing the straight-line distance between those points and the time taken between them, and calculating the average speed for those few seconds.

    The main speedometer measures the speed of rotation of the gearbox (transmission) output shaft and by applying a calculation based on the wheel and tyre (tire in US) rolling radius displays the speed at the wheel contact patch. It is also governed by rules that don't allow it to read less than the true speed, so it will generally be designed to read slightly high (maybe 5%). This is so you do not break a speed limit unknowingly due to an inaccurate speedometer. As your tyres wear, or if they are under-inflated, the rolling radius reduces, and therefore the inaccuracy increases.

    Because of these differences the sat-nav will usually display a lower speed than your speedometer. On a long straight road with good traction at a steady speed the sat-nav will be slightly more accurate than the speedometer. But during acceleration or braking the sat-nav will take longer to recognise the change in speed, and on twisty roads its methodology of measuring the distance between two points leads to under-measuring the distance travelled and therefore calculating a lower than true speed.

  • Anonymous
    6 days ago

    WTF is a speedometer bar? You can't read an analog speedometer accurately enough to compare it to the digital one.

  • 6 days ago

    Hey Dufus...the seat has a weight sensor in it. Something like 35 lbs. turns on the passenger air bag. Put your junk on the floor if you don't want the air bag turned on.

  • Anonymous
    6 days ago

    Time for a new car. 

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