Tyre Diameters

[Rob's Lexx]

not too sure on this one but i'm sure you could explain......

i have 215x45x17's all around on my car, what tyers am i able to upgrade to. by this i mean, am i able to have a lower profile without changing the wheel radius (i.e 17's to 18's). if so, could someone please explain why this is so.

also, what is the reccommended tyre size/profile for an IS200....i heard that some people run with larger tyres at the back than the front (or vice versa, can't remmeber which!!!) why is this????

cheers guys

[RX-Men-8]

Hi,

If you increase the wheel diameter and have the same width and height of tyre, the entire wheel+tyre size will get bigger. This will affect the speedo/tachometer (lower indication) or will not fit the wheelarches at all. So you need to go to a lower profile tyre.

Roughly, if you increase the size of the wheel by one inch, the profile height percentage has to be lowered by 5 (this is appprox. for 225 mm tyre width)

So.... 205/55R16 -> 215/45R17 -> 225/40R18 -> 225/35R19 have about the same circumference.

Why wider tyres at the back?

- Rear wheel drive may have use for the extra traction

- There's More space in the wheelarch as rear wheels don't need the room for steering

- Looks muscular like a hot rod.

Wider tyres mean a lowe profile (percentage) as well, e.g 225/40R18 and 255/35R18 have the same circumference. On the IS this is a nice setup for front and rear, respectively.

Cheers,

RX-Men-8

[ColinBarber]

If you put on a larger width then you can have a low profile number, however the actual height will remain the same as the profile is a percentage of the width.

Just changing the profile will result in a smaller wheel/tyre circumference and therefore would affect your speedo/odometer.

[Rob's Lexx]

right, kinda half got ya.

but i don't understand why even if you increase the width of the tyre but decrease the height (profile), the circumference remains the same. not sure how just the width can help keep the cicumference the same??

on this basis, i have the standard 5-spoke lexus alloys with 215/45/ZR17 (wat does ZR mean aswell???) all around. what could i put on instead of these, without having to adjust/change the alloys (coz i like the alloys i've got)?

[RX-Men-8]

The profile number is a percentage. A wider tyre with the same profile percentage hence also means a higher tyre.

Example 215/45ZR17.

So 45 means the tyre height is 45% of the width, at 215mm width the tyre height is about 97mm.

If you take 225/45R17, the tyre height is 101mm, resulting in a bigger circumference.

Tyre size calculator

Cheers,

RX-Men-8

[Rob's Lexx]

The profile number is a percentage.  A wider tyre with the same profile percentage hence also means a higher tyre. 

Example 215/45ZR17.

So 45 means the tyre height is 45% of the width, at 215mm width the tyre height is about 97mm. 

If you take 225/45R17, the tyre height is 101mm, resulting in a bigger circumference.

Tyre size calculator

Cheers,

RX-Men-8

←

oh right, i'm with you now!!! oi always thought that the middle number (e.g 45) stood for height in mm!!! no wonder i couldn't understand it all!!!

thanks for your help matey.

by the way, wat does the ZR mean??

and what set up would you reccomend??

[ColinBarber]

Z is the speed rating - > 167mph. You only really need a V rating on the IS but most manufacturers only make a Z tyre in that size.

R means the tyre is of a radial construction - all modern tyres are this type.

[Rob's Lexx]

cheers colin for explaining that!!!

can anyone tell me what tyre set-up to use (i.e what you would reccomend to have on the back and the front)

with the standard alloys that are on it, can you still fit a wider tyre????