You could simply close off your carburetor's idle circuit. There's a handy needle valve (aka idle adjustment screw) to do it. Once done, your car would not only shut off when going down hills, it would also shut off automatically at stop signs and stoplights, just like the hybrids.
Of course, there are the minor problems that many times you want to have the engine idling, and you'll waste a lot of gas starting up at high rpms. 
__________________
Darrell
Boycotting Exxon since 1989, BP since 2010
Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac? George Carlin
Mean Green Toaster Machine
49.5 mpg avg over 53,000 miles. 176% of '08 EPA
Best flat drive 94.5 mpg for 10.1 mi
Longest tank 1033 km (642 mi) on 10.56 gal = 60.8 mpg
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