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I remember many years ago, researchers were able to get phenomenal gas mileage (for the times) without any major changes to the vehicle. They would disconnect the accelerator pump on the carb. (Maybe the power valve, too.) They would accelerate in the highest possible gear at full throttle until they got up to 45 mph or so. Then they would coast down to 30 mph or so and repeat the process. Now this goes against the conventional wisdom that accelerating and deccelerating is bad for gas mileage, but actually it makes perfect sense: An internal combustion engine is most efficient at wide open throttle. What kills your gas mileage when you accelerate a carbureted engine is the enrichment circuit in the carb. Now, to answer your question. Obviously, when you go downhill in low gear, your engine is converting the energy of your vehicle into heat. You will get better gas mileage coasting, but only if you don't use your brakes, and assuming you don't crash. For safety's sake, don't coast. One risk of coasting is that the engine can die, leaving you without power steering or power brakes. The older carbureted engines would suck quite a bit of fuel going downhill in low gear, because the idle circuit is fairly rich. Hence the backfiring going downhill if you have an open exhaust. Newer electronic fuel injected engines keep the mixture optimal at all throttle settings.
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