The usual guideline for highest cruising speed is 2,500 feet per minute piston speed.
F/M = stroke × RPM ÷ 6
2,500 F/M = 15,000 ÷ 4.12 = 3,641 RPM

Highest sustained speed is 1,500 feet per minute.
1,500 F/M = 9,000 ÷ 4.12 = 2,184 RPM