Tyre Change from 40 to 45 profile

[Scamper]

I am currently running 255/40 17" on the rear of my 964 C4 what are the implications of changing to 255/45 17" profile tyres?? will they fit on the rims for a start, how will it affect the handling?

I dont know much about tyres just usually get the exact same sizes when I change them....

Thanks

[LeoSayer]

The rear of the car will sit around 13mm higher on 255/45 thereby raising the centre of gravity at the rear. I imagine you will also knacker your 4wd system very quickly.

They will fit on the rims, but they may rub on the wheel arches.

One question...why?

[ian964]

Also, if there's too big a difference between front and rear rolling diameter on a C4, you'll have all sorts of ABS and 4WD problems. If you're running tha standard 17" fronts (205/50 x 17), the rolling diameter is 636mm. 255/40x17 are 638mm, but 255/45x17 are 661.

Why do you want to do it?

[Scamper]

My tyres have about 2k miles left in them so its not a question of needing to get them done. I was just curious as to what the effects might be.

I have read a lot (on the internet) about reducing the profile, increasing the width, etc but never heard of anyone increasing the profile - obviously there is a reason for it, its just that I havent heard why its not a good idea.

I am currently running 225/45 17" on the front and 255/40 17" on the rear. This seems to be a common tyre set up.

[LeoSayer]

I see what your asking, but I think it's a complete non-starter.

You would need to change the front tyres to 225/50/17 or 205/55/17 to get the same rolling radius as the rear on 255/45/17.

For a start, I doubt that you could find Porsche 'N' spec tyres in that size and even if you did, an insurance company may refuse a claim if the tyres were the wrong size and you crashed as a result.

The tyres would have thicker sidewalls so, in theory should give a better ride, but the downside could be more sidewall flex in corner so less precise handling. Precisely the same as you would notice downgrading from 17" to 16" wheels.