SOrry for not replying before 
well, if u ve driven 8000kms in 3yrs... i would say u shudnt even consider the fuel consumption
just enjoy the grip every time u drive... theres no real reason to assume that mileage wud increase by 2km/ltr if tires are changed, yes it will certainly rise but may be it would be a negligible difference.
i'd say until this car of urs is transfered from being a 2nd or 3rd car to the primary car, dont get the tires changed. i checked the market 2-3 days before, yokohama DB 195/65/15 was for 8500rs/tire. 185 size wont be much cheaper...
that is what i wud ve done if i was in a similar scenario... now u gotta decide...