(T) Wheel and Tyre Size ?

OK.. guys dunno if anyone will know this as its a bit of a wheel expert come mathematicians question..

If i were to replace my 17 x 7 Alloys with 215/45s, for 18 x 7.5 alloys, What size tyres do I need so that it wont affect the speedo etc ?
Is it possible to get tyres with a profile that low ?


There's a PM to a mod waiting for anyone who gives a good answer
And if the answer comes from a mod then i'll just kiss your butt

Hi KoArAnG,

Been looking at putting mine on 18's, have a mate that deals in tyres & he suggested 225/40 R18ZR Potenza's

Dosen't think that the rolling radius of these tyres should make much difference to the speedo...... told me to drop a tape measure round my original tyres radius, the same with the new ones if the difference less than 3" makes little or no change to cars settings

Hope this is of some use matey........regards Steve

Don't know the answer, but you thinking of changing the wheels again?

I had a wicked link to a website that defines and tells you everything about tyres and wheels. It also has a tyre radius calculate and tells you what the difference is when u change the profile and wheel size.

I will post it when I get home from work.

Thanks Froggie thats great.. I will try that tape measure tip..sounds logical..
lol.. i know im a spending freak Not yet though James.. Just something im looking into for when I do need to change the tyres.. because I got these wheels for free as such, im tempted to get some 18s or maybe 19s if the stilo can take them ?
I would imagine if I went for the 19's i would need to get the speedo and trip altered to compensate ?

Nice one Vimto.. I will look forward to that

This is what the tyres guide from rochfordtyres recommeneds for each wheel size that fits the stilo! I believe these are rite sizes so that you dont have to have your speedo ajusted and trip computer reset, etc, etc..

http://www.rochfordtyres.co.uk/Fitment.asp?psearch=Fiat

15" 195/65/15
16" 205/55/16
17" 215/45/17
18" 225/40/18
19" 215/35/19

I found it!!!

http://www.chris-longhurst.com/carbi...yre_bible.html

A definate read for everyone

KoArAng, I posted you a lengthy answer to this question on a previous thread, detailing how to work it out etc.

The 215/45 on a tyre is the width in mm and the percentage of the width that the wall height is, so in this case the wall height would be 45% of 215mm or 225 x 0.45mm = 96.75, in effect it would probably be 100mm.

To obtain the overall radius of the wheel and tyre you could convert the 17" diameter of the wheel to millimetres by multiplying a 25.4, i.e. 431.8mm. In effect it is likely to be 425mm as they tend to use metric inches, if that makes any sense, but 25mm is a metric inch. So the overall width of the wheel and tyre in this scenario would be 425mm + double the tyre wall height (200mm), which would equal 625mm.

You would then have to work back from 625mm to calculate the tyre size for an 18" rim. So 625mm - 450mm (wheel size) = 175mm. 175mm would be double the required wall height, so the new wall height would be 87.5mm. On a 7.5" wheel you would be likely to use 215 or 225mm tyres, so you would need to determine what percentage of 225 for example would equal 87.5mm.

87.5/225 x 100 = 38.89%, in effect 225/40, would do the job.

To double check 225 * 0.4 = 90mm

(90 x 2) + 450 = 630mm, which is close enough for legal tolerances.

An effective difference of = 0.8%.

In the real world you may actually find the total diameters are identical as most maufacturers would manufacture tyres to similar overall diameters.

Yep I know you did Neofolis... lol.. and I was just as confused after reading that one as I am this one I needed it in laymans terms..in other works, im a thick b****r and everything you just wrote went through my eyes, swirled around the cavernous space between my ears and then dribbled off the end of my tongue as it fell out of my gaping mouth.. .. seems to happen a lot to me..
You wouldn't believe I actually design 3D game worlds for a living..
Shows you don't need to be a mathematician to get a job in gaming.

Or you could use the calculator in the link given above

Going by the rochfordtyres sizes that they recomend, if I wanted 19"s they are pretty close to the original radius of the wheels that i have now. After looking on the site Vimto said and using their calculator thingy it all seams pretty simple..
Thanks for your help guys.. very much appreciated

Problem is..would it look kinda weird with 19s on if you could see the original brake discs through them.. Maybe i would have to get a less open design wheel where you can't see too much through them.
like the OZ superturismo GT's

Do people really bother to look at the brake discs and think, ooh that looks a little small. It's fairly obvious once you have a tyre profile that low that the wheels have been changed for a larger than standard size, so what the hell.

I don't know what brake discs you have...but EBCbrakes have some cheap turbo grooved ones aswell as the greenstuff brake pads. Also might be an idea to paint you calipers blue or red...looks better through alloys.

Yup, definitely