[Ptero]

RIDING THE CLUTCH
All transmission bearings are cooled by an oil bath that draws heat away and replenishes lubricant. A transmission could care less whether you are in neutral or in a gear. But all throw-out bearings are sealed and self-contained. They have no provision for cooling or lubrication replenishment. Therefore, riding the throw-out bearing for extended periods gets it hot. When the throw-out bearing gets hot, two things happen: 1) the grease thins and 2) the oil seals soften. There is a point where the hot, thin grease can make its way past the soft oil seals and escape. If too much grease is lost over time, the throw-out bearing will seize. When this happens, the outer race of the bearing, which is usually stationary, will spin on the clutch forks. This will wear the clutch fork, causing the clutch to fall out of adjustment and necessitating replacement of the fork. Even worse, the inner race of the throw-out bearing will drag on the release fingers of the pressure plate. Eventually they will wear away or break off, throwing debris into the clutch/flywheel/pressure plate interface, scarring up the flywheel. This requires flywheel removal, resurfacing or replacement.

My experience over many years as both a mechanic and heavy truck driver is NEVER hold the clutch down for longer than it takes to shift - or you will eventually be paying a mechanic all the money you saved (by coasting with the clutch in) to repair your clutch assembly.

COASTING IN NEUTRAL WITH AUTOMATIC TRANSMISSIONS
Never do this. Always "coast" in drive. If you have been doing this in a car that the manual does not specifically say can be "flat towed," you should sell your car.

COASTING IN NEUTRAL WITH MANUAL TRANSMISSIONS
In heavy vehicles, coasting in neutral is extremely dangerous and should never be done. In lightweight vehicles, coasting in neutral has little significance as a control issue and provides the greatest mpg gain possible.

I have reviewed the posts on this thread and would like to point out that it does not take much fuel to idle an engine that is not doing work. If you are coasting, you are receiving a tremendous advantage regardless of whether or not the engine is running. Of course, there is an advantage to having the engine off altogether, but on modern cars this disables other systems like power brakes, power stteering, airbags and brake lights and is therefore not an option. Also, you do not want to pop a dead engine back to life at speed because it is stressful on the components and wears the clutch excessively. It is better to start the engine with the key, rev the engine to match the transmission engagement point and engage. Neophytes can locate this point by very lightly touching the shift lever without engaging the clutch, delicately and slowly revving the engine, and feeling the teeth in the transmission slow down as they approach the mesh point. NEVER engage without tapping the clutch until you get really good at this, for two reasons. One, imperfect matching will wear the brass surfaces of the synchro rings excessively and, two, you can break off a tooth which can destroy the entire transmission.