Jeff@Longo

Hey guys,

I received a call (well, a message) regarding whether or not 20" wheels (from a GS) will fit the 2nd Gen IS. Here's my guesstimate:

The provided tire size (optional 18") is 255-40-18.
Upon calculation, it shows a 10.2 cm (4.02 inch) per sidewall + 18" rim. The size is to 26.04 inches for the oem 18" wheel.

For a 20" wheel to fit (w/o gaining/losing sidewall profile), you'd only have a 2.02" sidewall (5.13 cm). Assuming 245 width, you'd have to run 20 aspect ratio (lol, does that exist for 245 wide?)... so it seem that it will not work.

Of course, this is all based on numbers w/o increasing the overall tire size of 26.04 inches. The wheel to fender gap in the rear is about 1.5" while the front is about 2.25".

Hopefully that answers "Mr. X"'s question.

EDITED to include the other side of the tire for overall diameter of the wheel. The end result is the same though.

rominl

iTrader: (4)

haha jeff, one big mistake there you actually have 2 sidewalls not one. in fact the overall diamter should be around 26 inches (18 + 4 + 4).

and if you talk about overall diameter, it should be ok. if you look at the tire
255/40/18
255/35/19
255/30/20

they should have pretty much the same overall wheel diameter

Neo

iTrader: (3)

Can the new IS handle an extra inch in diameter? That is what the 2GS with 20s have to do.

Jeff@Longo

Oh yea... =P thanks!!

Sorry... you know how it is when you just get into work and get bombarded w/ questions and work =P

But eitherway, since the measurement was left out for both rim sizes, it'd be the same result. =P Edited the original to make sense hahaha. Thanks.

Since 255 is the width in question, we'll use that as an example. The general rule of -5 aspect ratio per 1 inch increase of rim size works on smaller width tires, but on 255, it doesn't.

255 x 5% = 12.75 mm = 1.275 cm = ~.5 inch You would need a -10 aspect ratio change to maintain a total diameter equal the stock size.

So it'd be (about)
255 40 18
255 30 19
255 20 20 (does this size exist?)
to maintain a total wheel diameter (rim + tire) close to the stock size... It's probably best not to exceed the total OEM wheel+tire diameter especially with the small wheel well clearance that is available.

NEO: I'll have to measure the actual wheel well depth. I'll try to get it done by Friday. I have to catch a PDI tech in action to do it =P We're supposedly getting another shipment tonight.

rominl

iTrader: (4)

well, if anyting you can always run rubber band to make 20s fit on the IS

to me the issue isn't about whether the wheel and tire can fit, it's about the ratio.

20s can fit on the m3 no problem, but personally i think it's too big

Neo

iTrader: (3)

I was just addressing the original question and not passing judgement on the appropriateness of 20s on the 2IS.

the_ez_rhino

so, im confused. will 20's fit on the new IS or not?

Also, do you think that 20's are going really make a difference in the speed of the car, or is it too little to be noticeable? I think 350+20"LD1's will = hot...... But I can't be havin my rims screw up my 0-60

rominl

iTrader: (4)

if the size for the stock tires are correct then yes, you can run 20s no problem. but would it look good on the car? i have no idea

and yes, it will more than likely make the car slower

SoCalSC4

Another thing to remember- the 18" package on the new IS uses taller rear tires.

I'm SURE 20s will work.

A 305/25-20 is the same overall diameter as the 255/40-18 rear tire. 20x11 anyone?
There isn't really a "short" enough front tire to be a correct Plus 2.

A 245/30-20 is .8" taller than the front 225/40-18...

We could upsize the rear to 285/30-20 which would be .7" taller than the OE 255/40-18.

I think:
20x8.5 245/30-20
20x10 285/30-20

I need to measure one of these ASAP!

D2-AUTOSPORT

Also, just as FYI there has been a influx of new tire sizes to fit 20" wheels to smaller cars. Pirelli has a 235/30, Nitto has a 225/30, etc etc etc

For example, this Breyton e90 3 series is wearing 20x8.5 / 20x10.5 with 235/30 and 285/25-20

rominl

iTrader: (4)

hahha, someone is getting very itchy on the new IS huh?