Well, its best you determine the figures yourself. But there are some useful rules of thumb:
- At FULL THROTTLE a 40HP 4 stroke will use about 11.5 L per hour [approx 1 US gal per 10 HP less 25% for the 4str]. I don't know what your top speed is but I'd guess its 25-30 knots? So a 22.5 L tank would give you a range of about 50-60 nautical miles.
- Most RIBs will CRUISE at around 1 L/nm; but very small boats/engines (ie. 50 HP or less) seem to be able to to do around 2-3x this. So I'd expect 45-65 nautical miles.
The general advice is to allow 1/3 to go there, 1/3 to come home and 1/3 as reserve. But the best advice is to fill the tanks - take it for a 20 mile trip (e.g. with the gps tracking distance) then top the tanks back up so you can work out the use. Bear in mind that choppy conditions, extra people on board, etc will all add to consumption.