@Averagesupernova The mechanical fuel pumps with a centrifugal governor have an idle speed setting. The top engine RPM is not set, but there is a maximum fuel setting, (to prevent black smoke). Those settings are screw adjustable on the pump.
The accelerator pedal, coupled with the hand throttle, sets the wanted RPM, that is then compared mechanically in the governor with the actual engine RPM, and the fuel is increased or reduced to match those two. The mechanical control loop is damped in a bath of diesel or lube oil. Full depression of the accelerator injects maximum fuel without governor control.
Averagesupernova said:
Nowhere in the USA does any diesel engine that propels a vehicle down the road at highway speed govern the RPM as set by a lever.
That is not true for many trucks with mechanical injection pumps. Look at the older diesel Toyota Hi-Lux and Land Cruisers. The driver depresses the accelerator pedal to hold the speed they want, they can then set the hand throttle to clamp the accelerator linkage, the governor will then hold that RPM, while the pedal stays where it was before the foot was lifted off.
Diesels today are more petrol-like in their response, and are often fitted with a turbocharger. Normally aspirated diesels use a compression ratio of about 20:1, while those with turbos use about 16:1, (sufficient to start). The turboboost on a 16:1 engine can push the power output and cylinder pressures beyond what is reliable. That shortens the life of the engine if it is used often.
Common rail fuel injection is now needed to meet emission control regulations, so mechanical governors have been replaced with an ECU and software cruise control. The governor is now in the software, the programming of the accelerator pedal position can be for speed or acceleration.