The way you drive will help more than any mods though. Use the gears properly and you should only need to change gear once when getting up to speed. Pull away in 2nd, then switch to 5th at 30 mph, then gently cruise to around 56 MPH for maximum economy. When approaching junctions, lights or going down hills, put it in neutral and coast.
I point out a couple of small things. One, coasting in Neutral or with clutch depressed means the engine control system will inject fuel to keep the engine running. If you coast in gear, the fuel injection will shut the fuel off completely since the inertia of the car keeps the engine turning. At best coasting in Neutral will be a wash.
And two, in many places coasting in Neutral is illegal. It isn't likely you would get caught, though.
Doing a little searching reveals that gasoline engines run most efficiently with near wide-open throttle. That being the case, a "jack-rabbit" start can actually use less fuel than a slow acceleration. Make your own decisions, but take Internet advise with more than a few grains of salt.
Ive been experimenting, depending how big a hill it is, its way more efficient to coast in neutral as the car accelerates rather than being slowed by the engine braking. I can travel 6 miles in neutral on a certain stretch locally!
>One, coasting in Neutral or with clutch depressed means the
>engine control system will inject fuel to keep the engine running.
>If you coast in gear, the fuel injection will shut the fuel off
>completely since the inertia of the car keeps the engine turning.
>At best coasting in Neutral will be a wash.
At least on a diesel, the engine-braking effect of staying in gear at higher rpms costs more kinetic energy than you save. So it is often best to be in neutral (unless you want to slow down, in which case you should stay in gear to use the engine-braking).
Since Diesels have no throttle (the intake is always completely open) they have very little engine braking anyway. That's why compression brakes (Jake Brakes) exist on trucks. I expect Diesels, like petrol engines, cut the fuel off completely on throttle overrun, just as petrol engines do, and inject fuel to keep the engine running when in Neutral. It still does not address the issue of coasting in Neutral being illegal in many places.
"Drink driving" is a new one on me. I have heard of "DrUnk driving." Please cite a source for that statistic. Four per hour comes to 35,000 per year, and last I heard that is more than the total people killed in traffic per year.