I know that the rear wheels and tyres on the new IS are slightly different size to the front, compared to the old IS, so I have a question, how much difference does it make fitting new IS wheels and tyres to the old IS without changing the tyres over?

it can be Done ive seen a few old models with the new wheels....you may need to change the tyre sizes though, just to keep the speedo calibration correct

what i'm asking is if I don't change the tyres (the wheels i've seen have tyres already fitted and are almost brand new) how far out will the speedo reading be?

what size are the tyres

[img]http://www.is200.nl/JAE2006/23072006(002).jpg[/img]

225/45w17 on the front and 245/45w17 rear

Rear wheels (including tyres) will have a 4.1% larger circumference and front wheels (including tyres) will have a 1.4% larger circumference.

i.e you will be travelling ~ 5% faster than what your speedo shows.

So the MK2 IS front wheels are bigger than the mk1 IS rear wheels . I remember my cousins M3 having rear wheels around 245/40 17. Wonder if the IS actually needs the width, as thats increasing rolling resistance a fair bit. Must be better for handling and braking as larger surface in contact with road, although wonder how they would react to potholes etc

bugger

This isnt a massive amount. Around 14mm difference in radius, bearing in mind there is a difference of 5mm with new tyres compared to tyres that are still within their legal limit.

Speed cameras allow for ~10% over the limit, hence 5% on your speedo would be fine, presuming your speedo isnt already out.

the site I checked for fitment stated the speedo reading compared to actual speed needed to be within 2.5% so I don't really want to risk nearly 5% thanks for the help with my query, much appreciated

[quote name='Rory' post='441309' date='May 28 2007, 07:13 PM']the site I checked for fitment stated the speedo reading compared to actual speed needed to be within 2.5% so I don't really want to risk nearly 5% thanks for the help with my query, much appreciated[/quote]


1. The speedo is out by 10% anyway right?When my speedo reads 80 MPH my GPS says 71MPH

2.It will keep your miles down

I never thought about that actually... back to the drawing board

I saw my car there

At the rear, the 17 and 18 inch standard wheels are offset 50.

You can put 235/45R17 or 235/40R18 (like mine) on without having to roll the arches. Even with 30mm lowered suspension. The speedo will be very close to stock.

If you want to use the IS250 tyres, which are 245/45R17 or 255/40R18 you'll need to roll/shave the rear arches slightly.

On the fronts, the 225 tyres will fit straight on, in some cases you'll have to stretch the plastic wheel well liners ab tiny bit (blowdryer).

Cheers,

RX-Men-8