Of course it all depends on how big a rib that 50HP is pushing, no of persons onboard, how hard you drive it, etc. A 4m rib, one person onboard, not being used at WOT may get round on 50l with plenty of unused fuel as reserve.
If you give details of size of rib, number of persons onboard, driving style, others more knowledgeable than me will be along to advise.
Rib Tigga an Avon SR4 Rescue with Honda BF50
MMSI 235910073 Callsign T123046