For planing boats, the all-up weight is probably the most important factor. Length, hull shape, prop and engine efficiency all play a part of course. Power: weight ratio is the main thing that affects speed, so for two similar boats using the same power, the lighter one will go faster or use less fuel. If people can have a stab at the all-up weight of their boat when they post on this thread, the answers will be more useful, e.g. my hull should be ..., carrying 100 litres of fuel, 4 people and one small dog, or whatever.
We have a graph of power versus speed for various all-up weights on our website: http://www.eco-marine.co.uk/xr-range.html
The figures are from a well known text book and apply to boats in general, not just RIBs. Fuel consumption is more-or-less proportional to power, so where the graph shows a heavy boat needs twice as much power as a light one for the same speed, it will use twice as much fuel.