Speedo calibration

[GordonTarling]

Been studying the threads about wheel/tyre upgrades and thinking of changing my wheels from 16" to 17" - doesn't look like it should be a problem. However, I can't recall ever seeing any mention of speedometer calibration being affected by wheel/tyre size changes, so how do you get over that? Maybe you just accept that the speedo will over/under - read?

 

[jimmy]

When you go up a rim size you have to go to lower profile tyre size in order to keep the rolling circumference the same (or within 2-3%). All speedos are slightly out anyway. There is a tyre size calculator listed here on the forum somewhere, do a search.

 

[Aletank]

Tyre Size Calculator CLICK ME About half way down the page.
Lots more useful info there too !!

 

[Will]

Yep, as Jimmy says it is all about making sure you choose the correct size tyres. I.e, larger diameter rim + correctly reduced sidewall height of tyre = same overall size. For example, and quite extreme to illustrate this, a 255/35/18 tyre is the same rolling radius/diameter as a 195/65/15. Even if there is a few mm it will not matter, that is the same difference as a new tyre and one that is well worn!

Hope this explains what is going on!


Cheers,

Willl

 

[Andy W]

For example, and quite extreme to illustrate this, a 255/35/18 tyre is the same rolling radius/diameter as a 195/65/15.

Also 225/45/17 (my new wheels) is the same rolling radius as 195/65/15 (my original alloys)

 

[GordonTarling]

Many thanks chaps - that page with the tyre size calculator is a real GEM! It'll take me a while to read it all though!