Help Speedo off 14%

[capumetu]

I have a 2000 Z3. I am restricted to 225/50 16 tires on front because of strut.
I want to put 235/60 16 on back as this should come very close to speedometer accuracy.

Before I buy the tires does anyone know if they will fit without trouble?

[Jim@tirerack]

They are way too tall and are not going to fit without rubbing. They would also throw off anti lock brakes and gear ratio.

[capumetu]

They are way too tall and are not going to fit without rubbing. They would also throw off anti lock brakes and gear ratio.



Thank you Jim. That was the point to throw off the gear ratio. I was hoping that they would work as I am greatly dissatisfied with a speedo that reads so far off

[Jim@tirerack]

I would just get the speedometer fixed. I think that big of a tire is a bad idea.

[Critter7r]

Is the speedo showing a speed FASTER than you are travelling?

At any given actual speed, larger diameter tires will make the speedo show a slower speed than smaller tires, because they will rotate fewer times per x distance.

Also, from BMW ...

"Most speedometers have tolerances of approximately +10%. A “speedometer advance" is necessary to compensate for negative tolerances in tire diameter, electronic controls, tire temperature, tire pressure, vehicle load and other factors. Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle. If you are using a known accurate GPS system as a reference speed, you need to use the formula below to calculate the maximum permitted speedometer advance, which is 10% of the actual (input) speed plus 2.4 mph. Example: Actual speed = 50 mph … 50 mph x 10% = 5 mph … 5 mph + 2.4 mph = 7.4 mph … Therefore, the permissible displayed speed is 50 to 57.4 mph. (Note: The displayed speed must never be less than the actual speed.)"