Generac 5550/8550 Wheelhouse Generator

The frequency of the output is directly related to the engine speed. Actually, as this is a two-pole generator, the frequency of the output is the engine speed—60 cycles per second (nominal) times 60 seconds per minute is 3600 cycles (i.e. revolutions) per minute.

The governor should maintain the engine speed (and frequency) fairly closely, within a few percent. Typically you'd aim for a bit high under no load (a line frequency of 62-63 Hz seems to be a common suggestion) and then see that it's not too far low at full or nearly full load (58 Hz, say). Voltage should regulate itself when the speed is right to be within ±10% or so. I would suggest a small load, maybe 100W or so, for the "unloaded" measurements since you really don't care too much about the output when you aren't using it and since no load could conceivably do odd things to the output. (In particular, noise on the output could well lead to misleading readings on a multimeter with a high impedance input.)

By the way, most appliances and even more electronics are not very sensitive to power line frequency. Heating elements and some kinds of motors (non-synchronous motors) Many are designed to work with both 50 and 60 Hz power (since much international power is 50 Hz). The biggest difficulty would be some clocks and timing mechanisms running at the wrong speed.

My unsophisticated answer: I think that if hZ is Low while VAC is High, there could be inaccuracy in the measuring equipment. I'm not familiar with Inverter GenSets, though. In my old world, more RPM is more hZ is more VAC.

It might be the meter, usually voltage is measured using RMS. Your meter may not account for that. Non inverter generators are dependent on RPM. A way to check RPM is to plug a clock with a second hand into the generator and another clock that is battery or plugged into the house. When the second hands match over time the speed is correct.