Some just count engine revolutions, and are calibrated to read hours assuming say 2300rpm. I think most GA types are in that category.
Are you sure these are called "hobbs" meters? From what I understand a Hobbs meter is simply a clock that starts counting when an electric current is applied to it. And then it counts "true".
Whether that electric current comes from the master switch being activated, or an oil pressure switch, or a landing gear switch, or an air pressure switch in the pitot-static system doesn't really make a difference as far as the hobbs meter is concerned. (Although it does make a difference to the pilots wallet.)
But a device that counts engine revolutions, and runs this through an (adjustable) set of gears so that it seems to run "true" at a certain RPM is called a tachometer, as far as I know.