nankangs are fucking awfull!! Heard hundreds of bad reveiws including them blow outs!
my boss bought some brand new alloys with brand new nankangs fitted and within a week he had a blow out on the motorway at 75mph with his family in the car. Road debris was not to blame nor low pressure as he checked it was a faulty tyre.

Luke,

Firstly, the correct size for the focus is 215/45/17 not a 40 profile.
Secondly, don't skimp on tyres! they are the only thing connecting you to the road!!!