Hi Gary,
There is a difference between the injector bodies, the fuel pump and the mechanical or electronic injector control.
The "old" Lucas injectors were opened and closed by fuel pressure squirted down each injector line by a rotating disk in the high pressure fuel line. Those disks (and the housing / drive) needed replacing on a regular basis - particularly as they weren't designed for low sulfur fuel and will wear even quicker now.
The nearest modern equivalent is the Common Rail Diesel engine (used by nearly everyone other than VW / Audi) where a common supply of high pressure fuel is injected by electronically controlled injectors.
The VW / Audi Pumpe Deuse injectors combine the injector and the high pressure pump in one "unit injector" - so yes these are more expensive than a CDI injector - but you don't have the cost of the high pressure system
So comparing the costs for a VW unit injector with different technology is a false comparison. Especially when a Pug 405 offered around 90 bhp? and the current Pug / Ford engine will offer 150 bhp and twice the torque, and use less fuel.
And on the subject of gearboxes and fuel consumption, my car has a 7 speed automatic gearbox and happily tows in 7th at 60 mph - and returns over 33 mpg when doing so.
If the car engine produces insufficient torque at the engine rpm for the gearing ratio / road speed, then selecting a lower gear will probably offer better fuel consumption.
But if the car manufacturer designed the engine to produce sufficient torque to support the gearbox ratio and road speed combination, then towing or driving in the highest gear possible is the most economical approach.
Robert
It seems to be the common assumption that driving in the highest gear that the car will pull is the most economical way to drive, it's what I always used to assume, but the evidence I have seen from our old 110hp Sharan tdi and our new S-max 2.0 tdci suggests otherwise.
A blowing head gasket on the Sharan forced us to travel using very low throttle openings letting the engine rev with low torque in a low gear to produce the power needed to move, we got home to find that fuel consumption was better than usual.
Travelling home in the S-max I was pleased to see the computer showing 60mpg, I then realised I was only in 5th so I selected 6th and reset the computer, maintaining a similar driving style the mpg fell to 54.
In a much more controlled experiment driving 750 miles across France at 80 mph 6th = 37 mpg, 5th = 39.5 mpg
55 mph 6th = 54 mpg, 5th = 60 mpg, 4th = 54 mpg
towing at 60 mph 5th = 27 mpg / 2100rpm
4th = 29.5 mpg / 2700 rpm
I somehow doubt that our car is a freak.
My conclusion based on this data is that, as most of us know, using too low a gear means that more fuel is used to simply keep the engine spinning,
If too high a gear is used, the cylinder pressure required to produce the necessary torque is so high that lots of this pressure is wasted once the piston reaches the bottom of it's stroke and this valuable presssure dissapears down the exhaust, this is why cars appear to give ~50% of the available power with just ~25% pedal travel, and flooring the pedal does not appear to make as much difference as it should.
With 236 lbft of torque our car will very easily hold its speed in 6th gear but it uses less fuel in 5th, it would probably hold 60mph in 6th gear towing if I bothered trying, it easily holds 60 in 5th but it does 2.5 mpg more in 4th.
There is an optimum gear ratio for efficiency, which is different for every size of engine and load applied, this ratio is not necessarily the highest that the engine can pull.