What do fuel return pipes do on a diesel engine?

Doug Black questioned:

Ok, I know what the fuel relurn pipes do in watch to persistent unused fuel back to the pump etc. Just replaced all my fuel return hoses on my 1.8tddi Focus as they were all cracked leaking and or not collectively.

How much fuel do they return on habitual driving, is it extensive, I have been being paid a excellent 40mpg from it when they were all naffed leaking and not collectively.

Am I inane to see a definate boost in fuel efficiency or is it not really that much?

Cheers
yes, the hoses may not have even been on (in fact the lat one from the injector to the pump wasn’t) so none in the run were persistent fuel i guess! I was subdue being paid splendid milage tot he gallon (more than my 200tdi landrover for sure) is this inane to boost additional! like greatly?

hows a car engine work in regard to fuel efficiency?

Rich questioned:

i watched a TV show days gone by where a driving lecturer was instruction how to get the best fuel efficiency out of a car. he confirmed that if your foot isn’t on the gas, then the injectors will be switched off and no fuel will be used? but if a car is idling it is subdue burning fuel right? so does the engine know the difference between idling when the car isn’t tender and when the car is in gear but tender below momentum and not life obsessed by its engine?