To some extent it depends on the hull
of your 38 footer.
A modern 41 will displace some 7-8 metric tonnes, an older design easily twice this amount. You need roughly 2 hp / t to drive the hull
at econo speed (where consumption per mile is lowest) this would mean some 15-20 hp times 200 grams say 3.5 liters per hour and probably at around 5.5-6.5 knots. This makes possibly 60+ hours on a tank = 350-400 Nautical miles on one tank.
Now the heavier boat will burn twice the amount and end up with half the range. Since you have a 53hp engine in a 38 footer I am lead to imagine that your boat is mid- or heavy displacement
To extend you range you can be looking for the rpm / speed where consumption per mile is lowest - sometimes it is actually rather slow and many skippers decide to shorten the range but go at slightly higher speed / spend less time on the passage
. If this is the case then your engine is matched well and it will be properly loaded which is always good for the longevity of the engine (diesels hate to be run under light loads).