Mini 1.6 D dressed in S-Works kit

[Vdubbman]

Hi all, several years ago, I bought a 2011 R56 Mini 1.6 D with full S-Works kit, in BRG livery and red leather interior, including brake upgrade and centralised exhaust, for my Wife. She commutes in it, is a very cautious driver (I know; doesn't fit the profile).The car looks fab and, for all the Go-Faster Goodies, is cheap on fuel. The running gear however, is another story. The wheels are 18" DOTZ WHEELS Mugello rims with Michelin run-flats. They tramline badly, the car bounces around like a go cart on uneven roads and she seems to go through tyres like water, despite being a slow driver. I am considering putting smaller rims on with regular tyres, that will hopefully last longer, give a more comfortable ride and improve the handling. We have AA cover that will cover flats, so my wife, who rarely drives at night, is at lower risk of being stranded. The question is how I should approach what rim size to go for 15/16/17"? Looks are of course important!
Comments would be helpful