What's the most misunderstood thing about car engines?
When you let off the gas pedal, you spend NO gas: Modern-day engines (anything in the last 20 years pretty much) will spend literally no gas if your foot is off the gas pedal while it's moving. It will use the fact that your car is in motion to keep spinning, not gas.
Yes this is true. A lot of modern cars now switch the engine off altogether when coasting, you'll notice cars like the new VW Golf amongst others, the RPM's will drop to 0 as it switches the engine off on long descents, and coasting up to junctions etc.