Time to change new tyres.

Can someone please confirm if width of tyres affects rolling diameter/speedo?

I hear many say only tyre profile determine the rolling diameter hence the speedo. eg. using 35 or 40 series profile tyres would have different speedo readings.

The online Miata tyre calculator result in different speedo readings when selecting different tyre widths. Eg. different speedo readings choosing between 225 or 245 width tyres with same 35 series profile.

For stock 17X7 -225/50, my 19s should be fitted with 245/35 for the most accurate speedo reading.

But I would like to get 225/35 or 235/35 next on my 8.5inch wide rims but none too sure about the accuracy of the speedo reading. :confused:

Can someone please confirm if tyre width is going to affect my speedo? Thanks

try this site should help you

http://www.miata.net/garage/tirecalc.html

He's obviously already been there:

The online Miata tyre calculator


But yeah, width is a factor. Not sure how it works, I've seen a formula somewhere but didn't pay attention. I always used to think that it was just wheel diameter + tyre wall, but somehow the width also works it's way into the equation. Maybe tyre wall isn't the full measurement from wheel to ground?

The 35 is the aspect ratio.

It means the height of the side of the tyre is 35% of the width of the tyre.

Therefore ... the wider the tyre with the same aspect ratio the bigger the overall tyre diameter so the faster you will actually go with the same speedo reading :D

Thanks. I guess the Miata Calculator is right. Width of the tyres do play a part in effecting the rolling diameter and hence the speedo.

ol fart is right. The profile is a percentage measurement and thats what affects the profile of the tyre.

try this calculator... http://www.wheelsmaster.com/rt_specs.jsp

if you're just after speedo differences it doesn't matter if you don't know the offset.

cheers,
.wook

Ah, thanks ol' fart I finally get it! I guess if I ever actually sat down and though about it, I would have realised that no way were my tyres only 60mm high...