gearing and revs?

[bill_gj]

Best fuel economy is achieved by running the engine at peak torque - yes.
Unfortunately, the speed at which peak torque occurs varies with the load on the engine.
Power curves are usually published from data at full power, as the power requirement is reduced, the speed at which peak torque appears chnges. It petrol engines is usually increases, I can't remember what happens in diesels.

So, if you gear a vehicle to run at, say, a 3000rpm peak torque value, unless this is full-throttle at that speed, the fuel economy will be something other than what the engine power/torque/specific fuel consumption curves suggest.

 

[Lawnmowerboy]

I've got a little facet electric pump. Will that do? So what will happen to the little lift pump? Bits of swarf washed through to the main pump?

 

[normspanners]

the low pressure pump is a rotory vane pump called the transfer pressure pump and they wear even with diesel, because low sulfer diesel really should be written low oil / lubrication diesel the rotors pull the fuel from the tank and pressure it to the the cam type high pressure injection pumps to be injected When the pump weaars it causes cavitation ie gaps in the fuel.
I usually use a facet red top with the highest pressure to pick up fuel from the tank and supply the rotors which then boost it up enough for it to run ok .