Cruise RPM is a function of gearing, not carburetion, unless the engine is intentionally overspeeded. Governing the max RPM is almost always done to protect the engine from over-revving in lower gears when the truck is pulling a heavy load. A lot of the older work vehicles sort of self-governed top speed by running out of power even on level ground. Just whisper "hill" and most of them had a chill! The Freightliner I drove foe a while was like that- - - -it had a top speed of around 65 due to final drive gearing, even when running bobtail- - - - -and the worn out 318 Detroit 2-cycle didn't pull very well, either!
Jerry
