Shutting fuel off whie driving.

[Benjamin Marsh]

I want to build a circuit that will cut the fuel off to the engine
when I have my foot off the accelerator.
ie, when going down hills there is no throttle so cut fuel completely
to save fuel. 
Basically it will look at revs and if >say 1500rpm and throttle
switch is open to say 'no throttle' and to cut the fuel supply
which I will probably do by use of a relay in the ECU injector input
off the -ve side of the coil.
Are there any problems associated with doing this?
Are there really any fuel savings associated with it?
Are there any commercially available units that do the same thing?

Thanks for your help
Ben