10-11-2021, 02:16 PM
Speedo error is a percentage.
From new to old tires may increase speed display by ~4% on a car tire...more on SUV/truck, depending on OD and tread depth.
So, if speedo accurate/calibrated with new tires and the circumference is reduced by 4%
30 would increase to 31.2
60 would increase to 62.4
90 would increase to 93.6
120 would increase to 124.8
Same with changing tire sizes or even different brand of same size. I replaced a stock tire (Metzler?) on my 2012 Bonneville with a Shinko of the same size and it would barely turn when on the center stand, resulting in higher speed than previously indicated.
Yes, you may argue other factors, such as tire pressure lower or higher than door sticker, variance in temperature, etc. But, the bottom line is that 4% error will have approximately twice the MPH difference at 120 versus 60.
So, checking speed with GPS or Radar at a few speeds can indicate % error.
If not practical to calibrate, just apply that % to indicated speed.
From new to old tires may increase speed display by ~4% on a car tire...more on SUV/truck, depending on OD and tread depth.
So, if speedo accurate/calibrated with new tires and the circumference is reduced by 4%
30 would increase to 31.2
60 would increase to 62.4
90 would increase to 93.6
120 would increase to 124.8
Same with changing tire sizes or even different brand of same size. I replaced a stock tire (Metzler?) on my 2012 Bonneville with a Shinko of the same size and it would barely turn when on the center stand, resulting in higher speed than previously indicated.
Yes, you may argue other factors, such as tire pressure lower or higher than door sticker, variance in temperature, etc. But, the bottom line is that 4% error will have approximately twice the MPH difference at 120 versus 60.
So, checking speed with GPS or Radar at a few speeds can indicate % error.
If not practical to calibrate, just apply that % to indicated speed.