bigger wheels/Speedo

[SIMO888]

Evening all I was just wondering if putting bigger wheels on my car will throw the speedo out at all as I have herd of this happening on other cars when they went up in size is this fact or myth or does the speedo recalibrate it's self with modern day cars

[BillyCorgi]

Changing the rolling diameter of the wheels will affect the vehicles speedometer.
Look at the link to the RMS site for what tyre size change is permissible.
http://www.rms.nsw.gov.au/documents/roa ... 9-rev4.pdf

Recently, the wheels/tyres on No.2 Junior's car were updated from 215/45x17 to 225/40x18.
This changed the rolling diameter from 625.3mm to 637.2mm. This was an increase of just under 2%.
As the speedo error at 100kph was 3% to 4% fast, the change actually compensated for the factory error and made the speedo more accurate?
Cheers!

[fedaykin]

Speedo doesn't re-calibrate itself. You should be looking at the tyre diameter.

Check this site out: http://www.carbibles.com/tyre_size_calculators.html

[Jup]

Bear in mind that in the example above, the change was 11.9mm. This TTBOMK is legal.

I wanted to go from 225/50r18 to 225/55r18 which changes the rolling diameter by 23mm and is not legal, therefore not insured.

Check your road laws, and do your homework before changing tyre sizes.

[SIMO888]

will the offset of the rims affect much aswell

[BigBCalais]

Offset won't effect rolling diameter (ie Speedo calibration)

Often though, lower offset=wider wheels=different sized tyres which will effect it