There's a thread about it somewhere on this forums, you can look it up for more concrete feedback.
On FC, the a/t will give you 6+ in the city while the m/t version can reach up to 9+km/l if driven prudently. Overall though, based on the what ive gotten from owners and some nissan people on alternator problems and stuff, i'm not too keen on the Serena and would consider a Korean diesel like the Starex TCi over it anyday. I dont know how the newer ones with the QR25 engine fare though.