Need help with aloy wheels

So i ve had my k11 tempest for about a year now and i absolutely love the thing. Ive decided to add some aloys to it. But i dont know what size would be best atm it has 14 inch steels. Would a 15 inch fit? Or would i need to stick with 14 inch aloys.
 
I put 14'' and is very nice looking and i do not lose any power. i used to had 13'' steels
15'' you will lose some power
Its your choice !
 
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