Going from 18"s to 16 or 17"

Hey guys I was thinking about going down a inch or two with my rims.. yeah I love how they look, but idk maybe I prefer smoother ride and better gas mileage for smaller rims. Also Im concerned with engine performance since the day 1 using the 18s I felt less aceleration and engine makes more "effort".
Note: Car came with 195/65/R15 wheels.
Any advices?

Best regards,
Dwight :D