17's are a good compromise in every aspect,
Thanks for the help mAte, I decided to just put 17" on my focus because it is lighter and 18" is not advisable in my country bec of road condition.
Because the bigger the rim is; the heavier it becomes
This weight is known as "unsprung mass"
Unsprung mass is made up of all rotating parts outside of the drive shaft.
So bigger brake discs, bigger alloy rims all contribute to a slight reduction in the ease of take off most noticeable on hill starts.
The positives of bigger rims is the reduction in tyre side wall,
The replacement of rubber with alloy makes handling much better on cornering and high speed as there's no squatting or wabble in the tyre compared to 15 or 16"
Bigger rims make the suspension work harder so lowering springs and big rims make the car push hard on the road resulting in optimum driving performance.
I've fitted 18x8 ST rims with eibach 35mm Lowering Springs and it handles really well can almost Scandinavian drift if pushed hard enough on roundabouts can control a slight rear slip.
But overall I'd fit 17" rather than 18"
The 18's cost me 270euro every 6-8 months replacing tyres,
The contribution of unsprung mass is a bit noticeable in Diesel engines as well.
The turning radius of the 8" wide wheels means taking wide angle turns when driving around small roundabouts or driving in multi story car parks.
However the positive side of 8" wide rims is the stability at high speed the experience feels the same driving at 60mph on 16's
when driving at 140mph on 18x8 there's no wabble like the 16's would at that speed.
I bought my 18's from Ford purchasing one per month cost me 475 euro per corner
the look great but cost a fortune to keep tyres on compared to 17" I wouldn't get the money worth if I was to sell them and go for 17s myself so I'm going to stick with them but if I was buying again I'd go for 17's