Saw a truck around town today with a ridiculous lift kit and chunky off-road tires that were clearly much larger than factory standard, and it got me thinking; if you install this kind of modification in a car, do you need to adjust the speedometer to compensate? What about the odometer?

My logic is the only absolute measurement the car has is how fast the wheels and drive shaft are turning, so presumably there is some sort of multiplier - 1 revolution = X meters - that is then used to show speed and track distance travelled, but that factor would need to change if the circumference of the tires did

  • frank@sopuli.xyz
    33·
    44 minutes ago

    Hi! I was a controls engineering in the automotive industry in the US for a while.

    Yup. You sure should! Some cars even have tire dimensions and quick selections of winter/summer tires for exactly that. Some cars make it much harder/impossible to do.

    Same with motorcycles if you swap sprockets of course (a common modification)

    Edit: seems bikes are a pretty mixed bag where the speed sensor is. Your mileage (and speed) may vary there

    • Toes♀@ani.social
      4·
      3 hours ago

      Wait, is this actually a thing?

      I thought it would use some other metric to measure speed.

      Would this be true for say a Honda civic?

      • LifeInMultipleChoice@lemmy.dbzer0.comEnglish
        7·
        3 hours ago

        Not sure why you got downvoted for asking a question, but yes. Most cars use a system of counting the rotations of your tires. So a larger tire will have a larger circumference and therefore change the distance traveled per rotation. Note this means if your tires need air, it will alter the speedometer as well, as the circumference is actually altered by such. (Smaller radius than when fully inflated).

        This also means if someone spins the tires the speedometer will show they are going fast, when they are actually moving much slower.

      • frank@sopuli.xyz
        4·
        3 hours ago

        Well there’s lots of ways to measure speed. Some use a worm gear in the transmission, some use a sensor on the wheel hub. But all of them take tire diameter into account, unless you count like GPS, which afaik (though probably some really shitty privacy invading car may prove me wrong) isn’t a thing for speedometers and odometers

        So yes, all production cars, I believe.