Do i have to change the rims if i want to put 15 tyres? will there be any effect on fuel consumption
Yes you can change the rims.
Yes it will definitely effect fuel consumption. Whenever you go beyond the manufacturer instruction, consumption will go up and average will be less.
plus the speedometer will give incorrect readings because of increased size. for example, real speed us 80 km/h and your odo meter will show you 70 km/h. the more the speed, more will be difference.