changing wheel/tyre size?
 

If you change your wheel size from 15" to 16" then the rolling circumference changes. This then alters the speedometer and odometer reading, will a T4 session re-adjust this anomaly if it is input into the ecu?

I only ask as there seems to be a variety of size wheels fitted to our cars and they cannot be all reading the same mph and miles travelled.

Any info would be interesting.:bowdown:

regards Dave,

If you increase the size of the alloy, you decrease the profile of the tyre to keep the rolling circumference the same.

There are a number of calculators on the www where you can enter your existing wheel and tyre sizes, and they will calculate the size of tyre required if moving from say 16 inch wheels to 18 inch wheels.

https://www.willtheyfit.com/

There is also a table on the forum, and probably in the handbook where they give you the tyre sizes required for the different sized wheels offered on the 75

If you change wheel sizes and keep the standard tyre profiles for our cars then there isn't any change in the rolling diameter so the Speedo reading is unchanged.

The only wheel size that changes the Speedo reading is the 18 wheels this can be changed by altering the ZCS coding but it's not really necessary as it's within the 10% of the normal over reading of the speedometer ;)

Russ