Does size really matter?

[porschecrazy321]

My 987 S has 265 40 18 on the rear and 235 40 18 on the front, could i swap these when ready for the slimmer profile of 265/235-35-18??,they are a more common tyre size and a lot cheaper!!!,i do not know a great deal about this subject and a google search still leaves me confused.
I believe the 40 profile is standard fit in which case will be for handling reasons etc,there are various posts around the net saying theres no harm going to the 35 profile,if this is so does anyone have these fitted? and if so is all well?
regard

[amusingduck]

Fitting a lower profile will change the rolling diameter, and the speedo will over/under read.

Plug your details into here to see how big the effect will be

[jamieduff1981]

Sometimes, changing the relative rolling radii of front and back axles can confuse ABS and traction/stability control systems although I have no experience of your car.

[skyrover]

jamieduff1981 said:

Sometimes, changing the relative rolling radii of front and back axles can confuse ABS and traction/stability control systems although I have no experience of your car.

It can also burn out viscous differentials on 4x4's

[jon-]

Insurance is the other one to look out for, technically it's a modification and will need to be declared.

[DottyMR2]

Lowering profile would require a step up to a slightly wider tyre to compensate and give you the same rolling radius.

Try http://www.etyres.co.uk/tyre-size-calculator and plug the dimensions in to work out what you'd need to get the same radius. You may find the tyres you want to fit are within a sensible +/- % of the original radius so as not to be an issue.

[nails1979]

That small changes wont break anything and speedo will only over read a tiny bit more than it already will do. The only thing you'll notice is a harder ride from having less tyre wall to soak up the bumps