To enjoy maximum fuel economy and avoid CNG nuisance (unless the car is already CNG), stick to stock tyres. If you desire extra grip, i suggest you stick to 13" rim size and add some good branded tyres in maximum width of 185. Another option is to upgrade to 14" rim size...it all bottoms down to what you prefer most, fuel economy or road grip....best of luck...
(i would personally recommend Bridgestone Potenza GIII tyres)