Originally Posted by Frosty
In a modern car, when you coast in gear (like to slow down or go down a hill), the ecu cuts out the fuel and the engine is driven only by the wheels (down to about 20 mph or so). I agree that it's harmful to slam the tranny down several gears to slow down in the hurry, but done right, it's a big gas savings.
I always try to slow down when I can by just letting off the gas and coasting down in gear. If I need to slow a little quicker, I might drop it a gear. Yeah, it bumps up the rpm's a little because of the lower gearing but you aren't using more gas. Coasting in neutral or with the clutch in saves fuel but is still uses some to maintain the idle plus you don't have the engine braking to maintain speed down a hill (I refuse to risk a speeding ticket in the name of mpg).
The way I understand it (and I'm not a car guy, this is the dumbed-down explanation I remember), the car always has to remain above a certain RPM to keep the engine going. When you coast downhill in neutral, the car has to burn gas all the way down. When you coast down in gear, as long as the car is above a certain RPM (unclear to me how the RPM happens if not burning fuel), the car doesn't have to use gas.