The ratio of the PTO is dependant upon the gear it has and is a constant that is stated as a percentage of engine speed. Most of the SM420 with the single gear Chelsea ran at approx. 46 to 64% of engine speed. That is from memory and may not be correct but I used a ton of these in the 60's and 70's on SM420's and CH465's. The old hydraulic pumps were not good for anything over 1000 RPM which is where they were capacity rated so I advise 1300-1500 engine RPM as maximum.
