I'm assuming you have the earlier model (built before 1969?) I'm seeing a 10 gal tank. (may not be consistant through production run, like everything else.) There are so many factors in figuring this out. Depends on prop, drive train, (on this model it's off center or something if I recall) etc. Talk to a Volvo mechanic who may be able to give you a rough estimate.
We are sailing a Contest 25 Build no 113.
What is the diesol tank capacity and water tank and the single cylinder Volvo usage per hour at normal cruising speed.