I'm in the UK, I looked at tyre dealer web-sites and they suggested 225/40R18 tyre size as an option. I picked a 2014 i30 diesel Premium off Autotrader as I know they have 17" wheels as standard. I then used the registration number to search tyre dealer web-sites for possible sizes.
At least over here you'd have to inform your insurance company that you've made this modification and they might increase your premium because 1) fitting big rims like this suggests you're a boy racer and 2) just for the hell of it. If it's economy you're looking for then you have to consider potential fuel saving (if any) vs. additional insurance, cost of buying the rims. and the higher cost of the tyres. If that size of tyre means buying cheap economy tyres rather than a premium quality tyre then the tyres may not last as long and/or have a higher rolling resistance. I'd expect the ride to be a lot harder and jiggly, the standard springs and dampers are probably optimised for higher profile tyres which contribute to the damping.
I wouldn't expect lower profile tyres to deliver better economy other than by having to pump them up rock hard. and they're more vulnerable to damage from pot-holes.
