When you lower the profile you have to increase the radius correspondingly - its not a problem
i.e. go up one inch in radius and down one profile level.
This will have a negligible effect upon the speedos accuracy, in fact less than tread and inflation pressures would nornmally imply.
VDC/ABS/EBD will be fine. They have to cope with much more dramatic changes than a teeny weeny change in rolling circumference that actually is in their operational range anyway.
The speedo will slightly underead by a very small amount.
There are plenty of charts on the net that will give you figures for this stuff.
Don't just go down one profile and keep the same radius, in this case the speedo would be significantly out and probably would be illegal.
I doubt if it would have a huge impact on VDC, and is certainly something you could use at track days to try and get more acceleration (effectively lowering the gearing, rotational inertia and unsprung mass (very slightly!) at the expense of some grip, and traction.
The lower profile will make the ride harder for road use, but will provide a stiffer sidewall for aggressive cornering which should mean the car is more controlled and responsive under cornering especially on relatively smooth surfaces.
Assuming that you are on 16" tyres at the moment then you would have to go to 18" to get 45 profile.
Assuming that you still have the poorish standard dampers this might be cause a bit of jarring for everyday use - the rest of the suspension would have been designed to assume some more compliance in the tyres.
Originally Posted by DPP143
Can anyone tell me whether lowering the tyre profile (I run 205/55/16) and want to reduce to 45/or50 ..... would be a problem..? and maybe what changes in handling i am likely to experience..?
I'd love to put on 17's but can't find any I like here in NZ