wheel size/ speedo changes

[RTBMotorsport]

A friend of mine wants to change the size of his wheels from 13' to 17'. I told him that the speedo would be out, but I did not know how much....How do you work out what the difference will be ?
Cheers,
Rob

[speedy_thrills]

RTBMotorsport said:

I told him that the speedo would be out, but I did not know how much....How do you work out what the difference will be?

Why not work it out using % increase of wheel diameter? The original wheel diameter was 76.49% of the new dia., correspondingly the wheel will revolve 23.5% slower.

*I may be wrong about this method working, also its actually the tire diameter changed that matter and not the wheel diameter itself.

P.S. the “a friend of mine” thing never works.

[Tank Slapper]

Try this calculator. Should do the trick.

[rolyrat]

As a rough guide, for every inch greater in rim diameter, tyre width should go up by 10mm and the aspect ratio shoud drop by 10% This should keep the speedo accurate, I think. Youd better do the maths though.
Roland.